Innovative Reservoir Pressure Management Technology Facilitates Safe Drilling in Pressure Depleted Reservoirs
Authors: Prasad R. S(Srini) , Del Mundo Fulbert , Litvak Michael L , Flynn James J , Edwards Stephen T 
Published: October 2014

The example field in the Gulf of Mexico has been on production for seven years. Natural reservoir pressure decline has altered the stress regime and reservoir sand fracture pressure in certain regions of the field. Company drilling practices have established maximum Drilling Pressure Depletion Limits (DPDL) which are based on geomechanical analysis and wellbore strengthening theory. These limits must be considered to ensure safe and reliable drilling of new development wells in producing reservoirs. As reservoir pressure declines with production, the pressure depletion at a future well's target location may exceed the DPDL for that location. The risk of this occurrence may be managed through production curtailment at neighboring wells. This would reduce the pressure depletion at the target well location. However, this action would result in deferred production and value loss. Production curtailment can be minimized by sequencing the wells to take advantage of the characteristics of the existing well stock and the reservoir.

A full-field reservoir simulation model (FFM) is one of the tools used to guide the well sequencing in order to honor DPDLs at proposed well locations and minimize production curtailment. Building a robust well sequence is a multi-week trial-and-error process. The process determines if the DPDL at a target well location is exceeded, then selects the existing offset producers that need to be curtailed, and the duration of the offset well production deferral to raise the target well location reservoir pressure. The arduous, lengthy process precluded extensive "what-if" scenario evaluation. The resulting drilling sequence would have required significant production deferral over a time window in the future to facilitate drilling of the proposed development wells. The challenge was to optimize the development well drilling plan to reduce, if not eliminate, the field production deferral while drilling all the new wells safely by honoring the DPDLs at the proposed target locations. The challenge was met by developing a new automated, systematic and efficient procedure. This entailed building a pressure depletion tracking and management algorithm into the reservoir simulator and coupling it with the company's existing proprietary technology for field development option evaluation and optimization. This novel technology development automatically tracks the pressure at each target well location in the reservoir simulation model, selects the offset wells/zones that require production curtailment, and calculates the curtailment duration for thousands of potential well sequences to drill the future development wells. The well sequences are then ranked on the ability to honor the DPDLs at the target locations and associated production deferral.

The technology has made a step change in the evaluation cycle time by automating the systematic investigation of over 2000 well sequences, many of which are not intuitive. Errors were minimized through a standardized process that also permitted a broader evaluation of alternative reservoir scenarios. It has allowed us to evaluate the impact of various uncertainties including: DPDLs, stress path re-pressurization ratio, well productivity, and reservoir connectivity. This technology allows one to either develop a drilling sequence of water injectors and producers which minimizes/eliminates the offset producer curtailment or minimizes the offset producer curtailment for a given development well drilling sequence. We were then able to make an informed choice in selecting a well sequence that best achieves the multiple goals of drilling new wells safely, leveraging learning, identifying risk mitigation strategies, and reducing production deferral and enhancing value.