Well Stimulation Evaluation in Horizontal Wells with Emphasis on Petrophysics and Rock Mechanics: A Case Study in Deep, Tight Carbonate Formation

Abstract

Tight carbonate formations with extremely low porosity and permeability depend on well-designed completion and stimulation treatments to achieve economic production. Acid fracturing, a relative cost-effective choice compared with propped fracturing, is widely used for carbonate stimulation. However, many factors contribute to the acid etching created conductivity, which is a key parameter for the success of acid fracturing. From a petrophysical perspective, depth-by-depth rock mechanical properties, stress distribution as well as the heterogeneous petrophysical properties (e.g. porosity and permeability) are important local information affecting final fracture conductivity. In this paper, we conduct an integrated evaluation for multi-stage acid fracturing in a horizontal well in a deep, tight carbonate reservoir in Tarim field, China. We perform multi-mineral analysis and estimate volumetric concentrations of minerals, porosity, and fluid saturations with conventional well logs. Since shear wave sonic logs are not available for most of the wells, we estimate rock mechanical properties (Young's modulus and Poisson's ratio) using effective medium models including self-consistent approximation and differential effective medium theory. Corrections including the impact of fluids are developed using Gassmann's fluid substitution. Besides, we estimate depth by depth permeability with empirical correlations. Core measurements are used for cross-validating the well-log-based estimates of rock mechanical properties, porosity and permeability. Horizontal stress distribution and closure stress field are generated using poroelasticity stress model with estimated Young's modulus and Poisson's ratio as inputs. We also perform variogram analysis on well-log-based estimates of permeability and obtain its correlation length in both vertical and horizontal direction to quantify formation heterogeneity. The estimated rock mechanical properties, stress distribution, and petrophysical properties are used as inputs to 3D acid fracturing treatment modeling. The simulated fracture geometry, especially fracture height, is highly dependent on stress variation. The modeled acid transportation in fracture is strongly affected by permeability correlation lengths. The study result shows that the conductivity created by acid fracturing under local high closure stress is insufficient for successful acid stimulation treatments.