Parallel Estimation of Surface Subsidence and ...

SPE 164609

Parallel Estimation of Surface Subsidence and Updated Reservoir Characteristics by Coupling of Geomechanical & Fluid Flow Modeling Wathiq J. Al-Mudhafer, SPE; South Oil Company-Iraq & Louisiana State University Copyright 2013, Society of Petroleum Engineers This paper was prepared for presentation at the North Africa Technical Conference & Exhibition held in Cairo, Egypt, 15–17 April 2013. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright.

Abstract Determining of reservoir surface subsidence is a crucial problem especially for soft and unconsolidated formations. For long time production with water injection and/or water influx, the pore pressure decreases with increasing in effective stress leading to reservoir compaction and vertical surface subsidence. In this paper, a Tank Capacitance-Resistive Model (TCRM) has been done to estimate the history of vertical subsidence in the upper sandstone member/main pay of South Rumaila oil field in Iraq. It is a mature oil field with around 58 years of production with 40 producing wells and it has also 20 injection wells located only at the east flank. The average surface area for this reservoir is 142 km2. The reservoir is modeled considering an infinite acting aquifer at the east and west flanks. A commercial reservoir simulator has been adopted parallel with the Tank Capacitance-Resistive Model to estimate the subsidence in very active aquifer. The Carter-Tracy water influx model has been adopted to calculate the water influx rate. The TCRM depends on the concept of continuity equation that considers the difference between the water injection and oil production as reservoir input and output, respectively. So, the simulator calculates the total water injection rate and water influx to be treated as water injection in the continuity equation. The results demonstrated that reservoir formation has plastic deformation because it has recurrence subsidence in the reservoir thickness. The current subsidence for this reservoir in the current time after 58 years of predict is 1.5 ft at the crest of the reservoir and it is approximately close to the local subsidence at each grid in the reservoir crest.

Introduction The extraction of oil and gas for long time causes a huge decline in reservoir pressure. This decline may result in a reduction in pore pressure especially if there is not enough acting aquifer to support the formation pore pressure. This will lead eventually to decrease in petrophysical properties such as permeability, and porosity. Therefore, decreasing in pore pressure with constant overburden leads to increasing of the vertical effective stress that results in reservoir compaction and thickness reduction and might lead to surface subsidence and then results in damage to surface facilities and wells, but also reactivates faults(1). This model has been used with fast reservoir simulator to estimate the reservoir subsidence for non-aquifer reservoir. It can be taken place in unconsolidated and soft reservoirs, especially in those with large pay thickness and shallow depths. The surface subsidence had happened in specific parts of the Lost Hills oil field in California(2) and it was up to 10 ft. Also, it has been over nine meters in the Ekofisk field in the Wilmington field in California(3).