Geomechanical Characterization of a Matured Deep Jurassic Carbonate Reservoir: Explaining stress effects on production induced Fault Slip and Reservoir Development.
Authors: Al-shamali A. , Al-Mayyas E. A. , Murthy N. , Verma N. K. , Al-Sammak I. , Zhou C. , Perumalla S. V. , Shinde A. L. 
Published: September 2015

The Jurassic carbonate reservoirs in Minagish Field of West Kuwait have undergone significant pressure depletion (up to 4,000 – 5,000 psi) over the last two decades. However, during the last few years at least two wells showed sudden and significant reservoir pressure increase despite no injection in the reservoir for pressure support. The asset team plans to develop these reservoirs with more horizontal wells in order to increase the reservoir contact and thereby productivity and reservoir recovery. However, drilling and deepening the infill development wells in this area is becoming increasingly challenging due to uneven differential depletion across the field. Unprecedented drilling complications including mud-loss, well kicks, and differential sticking are observed.

This paper discusses how a field scale 3D reservoir geomechanical model integrating all available data was built and used to evaluate the impact of production induced stress changes on reservoir behaviour. Furthermore it details how geomechanical characterization provided inputs for the field development planning. The dynamic 3D reservoir geomechanical modelling of this field integrated: the structural geological model, well based 1D geomechanical models, rock mechanical test results from core, production data, reservoir simulation model as well as selected petrophysical and geophysical data. This model was initially built at original reservoir pressure. After proper assignment of both stratigraphically verified mechanical properties and boundary conditions of far field stresses, the finite element stress simulator was utilized to establish a representative initial stress state within the reservoir and its surrounding formations. The history matched and future predicted reservoir pressures at various time steps were coupled to the finite element mechanical simulator to map the changed stresses and strains over the reservoir interval.

The finite element analysis helped to investigate the associated changes of the in-situ stress field, pore pressure and rock properties across the field and specifically around the planned wells in order to capture the 3D effect of reservoir depletion such as arching effects. This analysis improved the field development planning by integrating wellbore stability risk assessment, fault slippage and other related aspects. The 3D Geomechanical model also distributed the shear-to-normal stress ratios over the interpreted faults/fractures and explained the dynamic behaviour of certain faults due to depletion. Field scale distribution of in-situ stress changes provided inputs to risk assessment due to further depletion. Understanding the stress induced response of reservoir due to depletion helped to plan new infill wells in due consideration of geomechanical risks and production efficiency.

The 3D Geomechanical modelling approach demonstrated that it is technically feasible to incorporate the complexity of 3D geological structure of a reservoir, fault network and other variables within the in-situ stress field. Using appropriate modelling simulations with realistic in-situ conditions, it was possible to explain the behaviour of pressure in wells, faults and also wellbore stability risks.