Good day to everyone
I hope you, your families, and your friends are okay.
Here I am again seeking SPE experts' feedback from the reservoir point of view.
The same offshore reservoir that we are operating in Libya is still in depletion mode. No gas or water injection is being implemented yet. As the reservoir energy has a certain limit, we had to apply additional/outsource energy to reactivate some horizontal wells after any planned or unplanned well/field shutdown which were attempted to be restarted naturally without such operation.
The main reason behind this issue is the water, as we are dealing with a fractured carbonate reservoir, most of the fractures which were initially filled with oil and act as the main storage of production, have drained out, and as a consequence gas and water came to the wellbore (steadily increase in GOR and WCT). However, when the well stopped, the gas segregated back to the gas cap, and the wellbore filled 100% with water. Bear in mind, the reservoir pressure continuously gradually declined.
To the point:
For reactivation of the ''lazy wells'' different methods were discussed, such as the bull-heading (utilizing diesel, N2, gas well, or one of the high GOR produced wells), and the coil tubing gas lift (CTGL). Although the bull-heading method is quicker and has no considerable cost compared to the CTGL operation, I did not recommend such operation for the below reasons from my point of view:
- The success ratio of bull-heading (BH) operation has demonstrated a steady drop as per our experience. Special caution must be given to the very few success operations done in the past:
- Why did the BH operation fail using some oil producers and the gas well, where it succeeded by another oil producer?
- What were the lazy well production parameters (Pr, WCT, GOR, etc) at that time before the shut-in?
- Where are the perforation intervals located (which formation), in which formation? and how much of the perforation intervals were in the matrix, and close/in the fractures (%)?
- Bull-heading (BH) is to push back the water, which is in the wellbore to the reservoir. This water mainly comes from the aquifer and will enter to the good reservoir formations!
- Formation damage analysis of the low- matrix permeability low-pressure reservoirs penetrated by the integrated detail petrographic, petrophysical, pressure, and fluid composition analysis. The analysis should identify several key factors contributing to formation damage in wells and at least four damage mechanisms likely to be at play within the reservoir. An incompatibility between injected fluids and, formation water compositions, over-pressured conditions during the operations (pushing fluid back), formation contents and lithology in the secondary pore space, and a shut-in period could be the risk key factors contributing to formation damage in the subjected reservoir. Especially at the current low reservoir pressure which possibly does not help to ensure the rocking operations.
- The four possible formation damage mechanisms that can be identified to be active in the subjected reservoir are (1) water blockage; (2) solid invasion of pore channels; (3) deflocculation and mobilization of any chemical synthesis or the molecular biology fines leading to blocked pore throats; and (4) salinity shock leading to the deposition of sulfates caused by mixing of incompatible invasion and formation fluids.
So, it is difficult to say with confidence which of these is the dominant mechanism, but the low pressure and low matrix permeability of this reservoir suggest that it is particularly susceptible to water blockage, wettability alteration, capillary forces, and relative permeabilities curves. Thus, a reduction in oil production may occur and an increase in water cuts (WCT).
Therefore, I am looking for your feedback in this subject.
In the meanwhile, we have launched some studies and investigations based on our database (previous bull heading results, production analysis, rock mechanics, properties... etc).
Your contribution will be helpful and highly appreciated.
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Installing a gas lift mandrel in the tubing is not feasible because the 7'' casing is a sweet service that cannot resist the reservoir's sour environment. and from the previous reports it is apparently the Kf>Km or even closer, the reason we are still using the single phi single perm model (although it is not the proper one).
however, for time being I am thinking of using diesel as a bull heading operation because the pump will provide us with suitable pressure to displace as much as we can water back to the reservoir. then swap to utilize the gas from the gas well to displace the diesel and lighting the hydrostatic column (giving the reservoir pressure a better chance). this will allow a better drawdown effect and invite much of the horizontal drain to contribute to the production.
The question, that is still in my mind is: what type of damage could happen if the diesel would enter into the formation? or may it will help to clean any emulsion that may occur from the water that has been injected back into the oil leg?
Looking forward hearing from you
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Mohammed
Jean-Francois raised some interesting points.
Might I recommend pursuing these opportunities with other communities:
- The Production Community and the Artificial Lift & Deliquification and the Flow Assurance Technical Sections have a lot to offer on both the original question and the challenge of installing gas-lift for sour wells that don't have sour-spec production casing.
- The Completions Community and the Well Integrity Technical Section are the best places to tackle the thorny questions of Sub-surface Safety Valves in concentric completions and barrier requirements in offshore wells that are incapable of flow by the use of deep-set sub-surface controlled safety valves (just in case that the worst case scenarios were to occur).
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I have received some very useful feedbacks from some experts in the Reservoir community, and I have advised to re post my request here.
Your assistance will be highly appreciated
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Best regards
Mohamed Attayeb
IOR Task Force Committee Chairman
East and West Mabruk reservoirs supervisor
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Emails:
mohamed.atayeb@mabruk-oil.comTripoli – Libya
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