Critically-Stressed-Fracture Analysis in Naturally Fractured Carbonate Reservoir–A Case Study in West Kuwait

Abstract

This paper presents the methodology to identify critically stressed fractures, CSF, in naturally fractured reservoirs. A natural fracture is considered to be critically stressed if the ratio of shear and normal stresses acting on the fracture surface exceeds the frictional strength of the reservoir rock. The main objective is to identify the critically stressed fracture trends in the reservoirs in order to design wellbore trajectories that efficiently intersect theses fracture trends. In addition, geomechanical analysis for drilling scenarios under depleting reservoir conditions addressing well-bore stability is attempted to formulate drilling and completion strategy. Critically stressed fracture identification has several important implications on fluid flow behavior through naturally fractured porous media. It has been shown that fluid flow in fractured rocks is largely controlled by critically stressed fractures; therefore, critically stressed fracture analysis may enable to systematically identify producing fractures in the reservoirs that mainly produce through natural fractures. The CSF analysis includes the mechanical property characterization of the formations and the in-situ stress tensor description acting on the reservoirs. The fracture orientations from core description & borehole image log interpretation were used for the CSF analysis because fracture dip and strike are needed for the stress calculation on fracture planes. This analysis is particularly useful where several fracture trends are identified in a reservoir, with some trends more likely to be open and productive due to horizontal stress anisotropy. The case history illustrates application of CSF Analysis in conjunction with geomechanical wellbore stability analysis in selection of optimal well trajectory and formulation of drilling and completion strategy for producing a naturally fractured carbonate reservoir. Introduction This paper presents a case study involving comprehensive geomechanical characerisation of a Fractured Najma-Sargelu (NJ-SR) reservoir in an oilfield of West Kuwait (Fig.1) with special emphasis on critically stressed fracture analysis. The work includes rock mechanical characterization, in-situ stress tensor analysis, critically-stressed fracture identification as well as well bore stability analysis for drilling scenarios under depleting reservoir conditions. The overall objective of this work is to provide results that can be practically used in the field to ensure proper well planning and drilling practices, as well as for the selection of suitable drilling and completion strategies for production. Analysis Methodology and Results Static Mechanical Properties Static mechanical properties are the fundamental inputs for in-situ stress estimation, critical drawdown pressure calculation, fracturing tendency analysis, borehole stability analysis and mud weight window designs. These properties are traditionally obtained by conducting triaxial compression test in the laboratory; however, such measurements are routinely not carried out due to expense and/or limited availability of core material. Therefore, an analytical program was used to derive static mechanical properties from well log data and formation petrophysical description. The program is based on FORMEL, a constitutive model describing the microscopic processes occurring in a rock sample during mechanical loading.1 The program provides continuous representation of the formation's mechanical properties with depth, which is the ultimate objective in geomechanical characterization. Well logging and laboratory data from five offset wells (#B, #E, #F, #G and #H-1) were utilized to determine the static rock mechanical properties of the lithological column, covering NJ-SR formations. At the depth of interest (>10,000 ft), this formation in the field may be classified as moderate to high strength, as the average UCS (unconfined compressive strength) values for these rocks were found to be in excess of 6,000 psi. The log derived rock strength was corroborated with laboratory test data on a core samples from one well (#B) in NJ-SR formations.

Structures and Tectonics of Kuwait

“…5.13). This evolved to a transtensional stress regime during the Cretaceous that was associated with a NW-SE maximum horizontal stress direction (Al-Kandary et al, 2012;Perumalla et al, 2014;Richard et al, 2014;Verma et al, 2007). The final stress rotation event in Kuwait occurred during the Tertiary.…”