Topic 45: Managing the risks and issues associated with a depleting reservoir.

Faults i think are the most important issues regarding safety.

Drilling a borehole on a fault or close to it is something very dangerous. In subsurface exploitations (mines) it is the most important mistake it can be made, because it affects stability, and in all cases should be avoided. There is no excuse for the engineer if a well is drilled on a fault or near it regarding a mine, and i think in reservoirs has similarly great importance.

Faults,safety considerations: When an anticline is spotted geophysic research takes place. If faults exist in a certain area, drilling must be avoided there. Certain safety distances should be kept from faults in order stability and safety are maintained in high levels.

Regarding land subsidence i have heard about Ekofisk too, i do not know how it can be avoided. Safety is reduced in an important degree depending on the level of subsidence, especially if the oil production continues in an offshore installation. This is because of the waves, and the water height, so in all cases the initial design of a platform should predict such a case.

Formation damage is associated with land subsidence i think, so they both regard safety. If land subsidence takes place it is mostly because of the formation damage. When oil comes to the surface the pore spaces of the reservoir rock empty and the weight of the overburden rocks, causes the formation damage and land subsidence.

Ref: Ahmed,2000, Reservoir engineering handbook 

The issue of resevoir subsidence or compaction is fast becoming a recognised issue in reservoir engineering, whereas it was previously only dealt with when problems arised. Despite the environmenal issues with subsidence it can also lead to enhanced recovery, particularly in Alberta and Venezuala heavy oils and oil sands. Subsidence is commonly estimated using the following equation:

Dh=hCm Dp, Dhs=R h Cm Dp (1)

This clearly demonstrates that the compaction depends on the amount of depletion Dp, reservoir thickness h, and the compaction coefficient Cm, which is a characteristic of the formation. These should be acquired during geomechanical data gathering from exploration, allowing estimates on subsidence to be made well before production to at least indicate expected levels of compaction that may occur, from which environmental impacts etc can be made.

(1) - Settari, A - Reservoir Compaction (http://www.spe.org/jpt/print/archives/2002/08/JPT2002_08_DA_series.pdf)

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Toby Stephen MSc Oil & Gas Engineering

As some of the posts in this debate suggest, managing the risks of a depleted reservoir can be difficult and challenging and can cause many of the problems mentioned by Edwin. Shale gas exploration is also at fault for this.

I would like to focus on induced seismicity (tremors/earthquakes caused by human activity) which are the result of extraction/injection of fluids within reservoirs, and the challenges that Governments & Exploration companies face because of the risks associated with them. From the research I have done so far into the management of reservoirs, it seems as though (but let me know if I am wrong though) there is a lack of research into the mitigation of induced seismicity produced from fossil fuel exploration - which only produce small tremors.

There have been no reported fatalities from induced seismicity. As we have learned from Dr Tan’s lectures, unless there is a major incident with fatality(s) in a specific engineering technology/business areas, Governments and Energy companies will not invest in research to mitigate the causes of these tremors. Hence the possible lack of research in the area.

In the US alone there are 1000s of small tremors caused from induced seismicity - perhaps due to the country’s reliance on Shale Oil/Gas and use of Carbon Capture Storage in depleted reservoirs. Although the majority of these tremors are small, public concern in the US has grown. If the US is going to become the global leader in Oil and Gas production by 2020 (from Shale reserves) surely more research must done in this area?

Ref Committee on Induced Seismicity Potential in Energy Technologies - http://www.nap.edu/catalog.php?record_id=13355 Accessed 23rd November 2012  

Haroon Latif MSc Oil and Gas Engineering

As stated in the opening discussion to this topic the re-activation of faults by nearby reservoirs poses a serious issue with the depletion of reservoirs around the globe. These changes cannot be clearly indicated yet but the tendencies to their effect is most likely.

I would want to address this topic from the area of downhole equipment. During the modelling and characterization of a reservoir leading to its production downhole equipment such as safety valves, packers, basic sensors and casings are designed under specification to respond to certain accepetable limits. With depletion comes a reduction in pressure (in cases where naturally driven wells lose support e.g. an obstruction to an aquifer) which by the re-activation of faults might cause the reservoir model used to design these downhole equipment to fail. The risk lies in the uncertainty of these events such as earthquakes and the response of these downhole equipment which are placed to protect the rig as a whole. In the light of this safety issues such as risk of a well blowout cannot be taken for granted also looking at the possibility of a total rig collapsing. 

The major challenge so far that has sparked up reasearch is creating a new generation of downhole sensors which will be able to perform under escalated scenarios. So far they are no recorded fatalities that have arisen from this but it poses an area towards further research in reservoir management.

Ikechukwu Onyegiri

Msc Oil and Gas Engineering 

Reservoir compactation will reduces the poisson ratio between depleted rocks and also changing reservoir physical and chemical properties can give birth to threats as stated in the introductory statement in this topic.

A major risks arises from earthquakes as discussed by Haroon Latif and this has raised a warning amognst environmental stakeholders and safety bodies as regards ageing platforms which could collapse and cause spilling of oil into the ocean. An example is the ageing oil platforms along the Southern California coast which activists have said should be reinforced holding to the uncertainties of earthquake in that region.

Owing to all these threats future survey on integrity of oil platforms with relation to reservoir compactation and depletion should be considered though recent debates suggests that the industry doesn't really regard this a threat.

Ikechukwu Onyegiri

Msc Oil and Gas Engineering 

Two major reservoir stress effects arise from depleted reservoirs and they are [1]:

1.  A drop in lateral total stress and

2. A rise in effective stresses

These lead to a drop in the formation pressure in the depleted zone and an increase in confining stress leading to stronger rocks which makes drilling difficult and substaintially increase the risks of blowout during drilling and incurring a lot of financial risks into the CAPEX. This risks are elevated in high temperature-high pressure wells with multiple zones.

Though some benficial effects might arise on the reservoir itself such as fractures due to shearing in the dense rocks and increased permeability as in the case of the Ekofisk the technical risks and potential financial risks is still not fully understood within the industry.

[1]  Desseault MB, Bruno MS, Barrera J, 2001. Casing shear: causes, cases and cures. SPE Drilling & Completion Vol. 16 Num. 2, 98-107, June 2001

Ikechukwu Onyegiri

Msc Oil and Gas Engineering

One of the challenges related with risk while dealing with depleted reservoirs is when you have to drill through such a reservoir in order to access an area below it. There is a great probability for the drilling operation to cause hydraulic fracture and loss of mud circulation because of the sudden pressure reduction in the depleted reservoir. This situation can be managed with the early application of the appropriate mud weight.

Another issue could be the differential sticking of the drill pipe caused by the increased wellbore pressure compared with the depleted reservoir pressure. In this case the retrieval of the pipe may be possible with the decrease of the mud weight or in some cases it will require a fishing company to come and undertake this procedure.

While managing the mud weight in the wellbore to solve the above issues, there should not be a significant weight reduction otherwise there will be instability at the wellbore above and below the depleted reservoir with the possibility of losing the well.

References:
1 - Mark D. Zoback. Reservoir Geomechanics. Cambridge University Press: 2007.
2 - Schlumberger Oilfield Glossary. http://www.glossary.oilfield.slb.com/Display.cfm?Term=differential%20sti...
3 – Wellbore Instability: Causes and Consequences. http://hrcak.srce.hr/file/30197

Reservoir depletion is the drop in average reservoir pressure as a result of fluid production. As such, fluid flow rates drop significantly making recovery from the reservoir quite uneconomical. In cases where the reservoir is bounded by an aquifer, water influx can help restore reservoir pressure maintaining flow. Likewise, gas capped reservoirs can experience the same scenario as the gas cap expands maintaining reservoir pressure as reservoir fluids are being drawn out.
Reservoir compaction is one of the challenges that can be experienced in depleted reservoirs. This can lead to irreversible permeability losses, increased sand production, mechanical damage to the well and casing deformation thereby affecting integrity of completions.
Drilling and completions are hampered greatly in depleted reservoirs. With reduction in pore pressure, fracture pressure also reduces. As a consequence, lost circulation is bound to occur. Clay instability and differential sticking can also be encountered during drilling in pressure depleted reservoir. Managing such risks can be done in a number of ways and these may include:

Restoration of reservoir pressure by operations such as water flooding and pressure maintenance techniques such as gas injection. These delay reservoir depletion ensuring productivity.
Use of robust geomechanical models can also be vital in identifying compaction issues during field planning stages. Such models can improve one’s knowledge about the reservoir thus predictions can be made about compaction with its effects on production and integrity of completions. Thus during development, better well designs can be made while applying proper zonal isolation techniques as well as employing sand control technologies like use of gravel packs.
Dealing with uncertainties related to drilling depleted reservoirs requires accurate prediction and monitoring of pore pressure. This in tandem with geological information can provide the best means of interpreting any pore pressure changes thus its effects can be effectively catered for. Interventions can include addition of bridging materials to mud to counter fractures that lead to lost circulation.

Reference:
Hemphill T, Halliburton Baroid 2005, integrated management of the safe operating window: Wellbore stability is more than just fluid density. SPE 94732, available at onepetro

Dear Colleague,

Due to depleting in well pressure, water ingression is inevitable. For each barrel oil produced, three barrels of water are also produced [1]. These water could be a migration from water re-injected to stabilized well pressure or from nearby aquifer [1]. This according to Schlumberger sources may poise 10 basic problem from easily solved to hard.

And one may asked, what are the impact of water relating to this topic? 

1. Water from non oil-producing zone may increase total OPEX due to more expenditure should be put to mitigate the unwanted water production.
2. Water channelling behind casing could leads to well integrity issue.
3. Fractures or faults induced from water stabilizing the well is escaping to surface.

Then, how to solve this? To stabilizing the well pressure and integrity, water is injected into the well, thus, water control is crucial.

1. To address leakage such as casing leak, flow behind casing etc by mechanical solution, by patch things up.
2. To inject treatment chemical into well to patch up any fault induced from reservoir pressure depletion.
3. to consider alternative such as multilayer wells, sidetracks, coil tubing isolation and dual completion.
4. to regulate water pressure injected into wells.

Regards,

Anas Abd Rahman

1. Bailey, B., Crabtree, M., Tyrie, J. [et al] (2000). Water Control. Oilfield Review. Schlumberger. (Spring 2000) Available from:

http://www.slb.com/~/media/Files/resources/oilfield_review/ors00/spr00/p30_51.pdf

[Accessed on 27 November 2012] 

There are so many threats that pose a great risk to a reservoir thereby depleting it. Some of these threats have been mentioned in the previous post. All these threats pose a risk to the performance of the reservoir and there are various ways of managing these threats. The first is to try as much as possible to reduce the risk by identifying it at the very beginning and prevent it occurrence. If nothing can be done at the beginning, then you accept the risk and mitigate against it by taking precautions. Lastly, if the risk cannot be catered for by the first two approach, then we resort to insure against the consequences that may arise from it.
Name: Alabi Ochu Abdulraheem
Reg no: 51231595

With the continuous production of crude oil, the reservoir
gets depleted. This result to changes in the drilling operation window thereby
creating challenges of loss of circulation, depressurisations, shale failure,
loss of mud weight and discrepancy in the calculation of stress depletion
response. At this stage , production becomes much more  risky for the operators of such wells like the
Brent fields in the North Sea, necessitating them to look for options that are
not tested and approved and could be unsafe if the procedures are not properly analysed
and potential threats identified causing instability in the borehole.
 The
existing infill drilling method are posing challenges and hence underscores the
necessity to apply latest technological advancement like the expandable tubular
and casing-while drilling methods. The former permits a number of mud weight to
be used in different sections without losing hole size while the latter helps
prevent borehole failure due to pressure cycling.

Reference

Davison, J.M., et al.(2004) ‘Extending the drilling
operating window in Brent: solutions for infill drilling in depleting
reservoirs’, IADC/SPE drilling conference, Dallas, USA, 2-4 March 2004.Onepetro
[Online]. Available at http://www.onepetro.org/mslib/app/Preview.do?paperNumber=00087174&societyCode=SPE
[Accessed on 02 December 2012]  

A depleted reservoir occurs when there is a drop in the
reservoir’s original pressure as hydrocarbon is produced from the reservoir or
when there is a drop in the original amount of hydrocarbon reserve in the reservoir.

Reservoir depletion reduces the fracture gradient
beyond which mud losses would occur, so one could conceivably drill with lower
mud weight

 The main issues
and risk associated with depleting reservoirs include:

1.    Increased
water cut production from the reservoir which could lead to the onset of
corrosion for topsides facilities

2.    Potential
management issues of increased produced water and its disposal which if not
managed well could result in pollution of the aquatic environment when the
produced water is dumped overboard.

3.    There
is likely to be changes in the reservoir conditions and composition with
potential for H2S; which increases the potential for corrosion of
plants and equipment.

4.    The
increased water cut from a depleting reservoir could also cause hydrate in the
pipelines that could result in line blockage, resulting in excessive line
pressure and potential loss of containment.

Reservoir depletion is managed through enhanced oil
recovering measures including gas injection, water injection or with the use of
semi-submersible pumps.
.

http://www.naturalgas.org/naturalgas/storage.asp

Depleted Oil reservoirs have been sited as a one of the main places that carbondioxide can be stored. This carries with it the benefit of enhanced Hydrocarbon recovery from the well. Again by injecting Carbondioxide into the geological structure by the use of Carbon capture and storage technologies, the effects of global warming are reduced.

Though these benefits have been proven to be true by research, there is little known of the effect of the injecting Carbondioxide into depleted reservoirs for storage. But in a scenario where the injected Carbondioxide comes to the surface due to failure of the system, it will be disastrous. The lake Nyos disaster is an example of the effects of huge volumes of Carbondioxide being spilled out into the envrionment. That disaster alone resulted in the death of about 1700 people and 3,500 livestock. [1]. This same disaster or even worst could perhaps happen if there is a failure of the geological structure in which the carbondioxide is deposited. A failure could occure in the event of a severe earthquake whose epicentre is exactly at the site of the storage of the Carbondioxide or a  tsunami occurring at the site of the deposit of huge volumes of Carbondioxide. Such a disaster will be horrible.

Depleted reservoir may be very useful in the storage of Carbondioxide thereby saving the environment from the effects of carbon pollution but caution must be taken in choosing the site for this purpose because a failure of a geological storage structure may be disastrous.

 Reference

[1]http://www.carbonify.com/articles/carbon-sequestration.htm

[2]http://www.netl.doe.gov/kmd/cds/disk44/F-General/IEA_report.pdf

I would also like contribute to the
discussion above. It has been said that the stress changes in the reservoir
cause the biggest problems. I would like to add that it can also cause shearing
of the casing, which can lead to extra costs. There might also be a free gas
development in the reservoir, and this needs to be taken into consideration.

Risks and losses associated with
depleted reservoirs can be dealt with by using water- or oil based slurry. It
is often called LCM squeeze (Lost Circulation Material Squeeze), which creates
a mass matrix and holds the surrounding rock in compression, which result in
strengthening the wellbore.

Aleksandr
Poljakov, MSc Oil and gas Engineering

In this discussion, most people have focused on the issues related to the reservoir itself; but I've worked in an oilfield that was undergoing a decline in production of approximately 6% per year, and one of the major reliability issues has to do with the topsides equipment.  It's an economic "Catch 22"; the field is declining, so the budgets are getting tighter, yet the equipment is aging and needs to be replaced or updated.  The ongoing question is "How much do you invest in a field that is in decline?"  The equipment has exceeded it's design life, the vendors don't support the maintenance of it anymore, but it's still working, so why spend the money to replace it?

My point is that a significant risk in fields on the decline, is the topsides equipment, and their potential for failure.  A question which I have encountered time and time again is, if you have a facility in an aging field on decline, and the facility is full of outdated equipment, do you spend the millions and millions of dollars to upgrade the facility (everybody with industry experience knows that upgrading a facility is a lot more complex and expensive than building a new one) or do you keep running it until there's a significant failure?

This is where the risk assessment process really comes into play, and establishing a process for elevating risks depending upon where they fall in the risk matrix.  I wish there was a simple solution to this problem, because if I figured it out, I would be a millionaire because of all the oil and gas companies attempting to answer these questions. 

As Richard mentioned
above, due to the good seal integrity and adequate storage capacity of
reservoirs, they can be utilized in the disposal or storage of carbon dioxide. This
technology can be carried out by the bulk injection of carbon dioxide into a
depleted oil or gas reservoir, serving as a climate mitigation strategy by
reducing global emissions.  Research has
shown that after injection of carbon dioxide into the reservoir, there was no
undesirable effect on the soil, groundwater or atmosphere. However, in the long
run there might be challenges associated with this technology, one of such is
that injection into the reservoir might be impossible or tricky due to the
injectivity gap depending on the depth and pressure conditions of the
reservoir. This wouldn't be a problem if pressure has been maintained in the
reservoir.

Before embarking on
this technology, the possible issues that could arise should be taken into
account, and a suitable risk control strategy prepared, which would include
preventive and mitigation measures.

Reference

Reusing Oil & Gas
Depleted Reservoirs for Carbon Dioxide Storage: Pros and Cons

One of the main challenges and risks in drilling
operations involving depleted reservoirs is the small margins between the pore
pressure (PP) and fracture gradients (FG).

This implies the pressure difference between static and
dynamic conditions in the well is small, which put strict limitations on the
mud weight and annulus pressure losses.

Also there is the challenge of heterogeneous formations
which possess another challenge for drilling into such zones as there can be
layers of significantly varying permeability which may lead to different level
of pressure depletion in these zones as the reservoir is being produced.

However, the modern oil and gas industry is up to the
challenge that these reservoirs bring forward and advancement and innovation in
technology can help us manage the risks in drilling in these zones.

One such drilling method is Managed Pressure Drilling (MPD)
which can help to manage the  risks in depleted
reservoirs and make drilling operation safe and reliable.

So what is MPD?

MPD is an advanced form of primary well control
typically employing a closed, pressurizable fluid system that allows greater
and more precise control of the wellbore pressure profile than mud weight and
mud pump rate adjustments alone.

As opposed to a conventional open -to-atmosphere
returns system, MPD enables the circulating fluids system to be viewed as a
pressure vessel.

“MPD is an adaptive drilling process used to more precisely
control the annular pressure profile throughout the wellbore. The objectives
are to ascertain the downhole pressure environment limits and to manage the
annular hydraulic pressure profile accordingly”

The MPD has four main advantages that make it stands
out and help to resolve some of the risks and uncertainties during drilling of
depleted reservoirs.

1. MPD processes employ a collection of tools and
techniques which may mitigate the risks and costs associated with drilling
wells that have narrow downhole environment limits, by proactively managing the
annular hydraulic pressure profile.

2. MPD may include control of backpressure fluid
density fluid rheology may include control of backpressure, fluid density,
fluid rheology, annular fluid level, circulating friction, and hole geometry,
or combinations thereof.

3. MPD may allow faster corrective action to deal with
observed pressure variations. The ability to dynamically control annular
pressures facilitates drilling of what might otherwise be economically
unattainable prospects.

4. MPD techniques may be used to avoid formation
influx. Any flow incidental to

the operation will be safely contained using an
appropriate process ”

References:  

(1) SPE 2006 -2007 Distinguished Lecturer Series:   Managed Pressure Drilling:  A “new” way of looking at drilling
hydraulics…overcoming conventional drilling challenges.

 (2) Morten Kartevoll:, 2009 ,” Drilling problems in depleted reservoirs.

Whilst I agree with my fellow student colleagues who have dwelt on the geo-mechanical risks of depleting reservoirs, I wish to take a different approach and focus on the associated financial risks.

Firstly, the Non Productive Time (NPT) arising from lost circulation in a depleting reservoir poses a great risk of significant economic losses. An estimated average of 300 hours per well of NPT were reported at the Brent oilfield in the North Sea where such conditions were encountered [1].

Secondly, extending the life of a depleting reservoir also poses inherent financial risks. The same Brent oilfield required a significant financial investment to extend its life at a time when oil production was declining. It was understood from its Gas-Oil Ratio (GOR) that the field still contained vast gas reserves to warrant production. The resulting development is said to have cost the operator approximately £1.3B (1992) to refurbish and enhance the safety of the production facilities [1] and extend the field life to between 5 and 10 years.

1. http://www.onepetro.org/mslib/app/Preview.do?paperNumber=00087174&societyCode=SPE

Kevin K. Waweru

MSc Oil & Gas Engineering

I would liketo address the issues associated with depleting reservoir from the area of enhanced oil recovery EOR.

Depleting a reservoir is to exploit to produce hydrocarbon. The reservoir engineers have employed several vital tools to increase the reservoir sweep efficiency. These activities ranges from Gas injection wet or dry gas, water flooding, and steam injection, drilling of horizontal and multilateral wells to exploit hydrocarbon reservoirs.

As much as our hearts are in the economic benefits of optimizing reservoir depletion/exploitation we also have to recognize the impact of this operation
to health and safety. For example in the USA most homes are on top of oil wells[1] as directional wells cut across settlements in order to exploit reservoirs.
This is would perhaps be an accident waiting to happen in an event of wave movement in the crust.

The probable consequence of EOR includes:

More to this would be the contamination of underground water reservoir by surfactants/chemicals used for EOR activities.

Managing these issues would be to improve on the technology used for this process and observing safety standards and procedures in depleting a hydrocarbon reservoir.

[1] http://www.usallc.net/services/horizontal-directional-drilling.shtml