A Novel Approach to Estimate the Variations in Stresses and Fault State Due to Depletion of Reservoirs; Case Study

Geo mechanical changes may occur in reservoirs due to production and depletion of reservoirs. Study of these changes has an important role in upcoming operations. Frictional equilibrium is one of the items should be studied. Pre-existing faults and fractures will slide in regions there is no frictional equilibrium. Equilibrium may be lost due to depletion. Whereas, it is possible that production recovers the equilibrium in some regions. Achieving or loosing of equilibrium depends on in situ stresses and rate of production. Casing shearing or lost circulation may be occurred due to faulting. In reservoirs which depletion leads to frictional equilibrium, decrease of fractures and faults leads permeability to vary. Hence, predicting the effect of depletion on frictional equilibrium is helpful in dealing with casing shearing or lost circulation in drilling of new wells. In addition, permeability modeling will be more precise during the life of reservoirs. Estimation of necessary time to create faults or equilibrium the faults is vital to be successful in production or in drilling new wells. Estimation of the in situ stresses is the key to study the state of faults. The next step is to predict the effects of depletion on in situ stresses. Different models are suggested in which decrease of horizontal stresses are predicted as function of pore pressure variation. In these models, different assumptions are made such as: simplifying the poroelastic theory and ignoring passing time and the geometry of reservoir. In this article, a new model is proposed based on theory of inclusions and boundary element. This model considers the geometry of reservoir. In addition, changes of in-situ are obtained as a function of time. Finally, the model is imposed on a real case study. The effect of depletion on faults are studied in reservoirs of normal and strike-slip stress regimes. Previous and proposed models are compared. In the first step, Mechanical Earth Models of these two reservoirs are built using logging and coring data. Zoback polygon method and poroelastic horizontal strain model are used for strike slip and normal regimes, respectively. In reservoir 1 which is in a strike slip stress regime, a fault is distinguished in FMI log. For this reservoir, the required time to achieve to frictional equilibrium is calculated. In the reservoir 2, the potential depth of fault sliding is analyzed and required time for faulting in that depth is predicted. The predicted time for satisfaction of frictional equilibrium using the proposed model has a significant difference with the predicted time using the previous methods. In addition, the proposed model predicts that different parts of reservoir 2 are willing for faulting during depletion. The previous model determines only one point that faulting may happen. It is necessary to use this new model to consider different important factors such as geometry and time to gain more accurate predictions.

کلیدواژه ها:

Fault state ، Depletion ، Stress variation ، theories of inclusions ، Mechanical earth model