Geomechanics Optimizes Field-Development Strategy of Deep Gas Reservoir
Authors: Denney Dennis 
Published: February 2015

This article, written by Technology Editor Dennis Denney, contains highlights of paper SPE 108891, "Using Geomechanics To Optimize Field-Development Strategy of Deep Gas Reservoir in Saudi Arabia," by Mahbub S. Ahmed, SPE, Saudi Aramco; Thomas Finkbeiner, SPE, GeoMechanics International; and Adnan A. Kannan, SPE, Saudi Aramco, prepared for the 2007 SPE Asia Pacific Oil & Gas Conference and Exhibition, Jakarta, 30 October-1 November. The paper has not been peer reviewed.

Saudi Aramco has embarked on drilling long horizontal and maximum-reservoir-contact wells in deep gas reservoirs. The regional stress is important in predicting wellbore stability and production performance. A detailed geomechanics study was made, and its results were incorporated into the decision tree. The objectives were to find the optimum azimuth to place horizontal wells that would minimize the risk of hole breakouts, optimize drilling-mud weights, and ensure sustainable production under depletion mode.

Introduction

The subject field is a north/south-trending anticline. The target reservoir is the Permian Unayzah formation, which was discovered in 2000. The reservoir has three geological units: Unayzah-A, siltstone, and Unayzah-B.

Because of stratigraphic considerations and constraints on wellhead placement on the surface, field development with highly deviated horizontal drilling is required to reach the target reservoirs. Reservoir management decided to extend the horizontal wells as far as possible to maximize reservoir contact and, hence, production from the Unayzah formation.

Because of the highly deviated/horizontal nature of the planned development wells, it was decided to gain a better understanding of whether geomechanical processes might be of concern to wellbore stability. If so, could well integrity be maximized by use of correct mud weights and well directions that would ensure stable conditions during drilling and production?

Determining Stress Orientation Approximately 2,500 ft of electrical wellbore-image data covering the Unayzah-A, -B, and parts of Unayzah-C reservoirs from the first four vertical wells drilled in this field were analyzed. The image data were calibrated, verified, and dynamically normalized to provide optimal image quality. Overall image-data quality in all four wells was fair to good. This analysis identified and characterized stress-induced wellbore failure, such as stress-induced borehole breakouts and drilling-induced tensile fractures.

Wellbore breakouts are enlargements of the wellbore wall with 180° spacing caused by localized shear failure, where the circumferential hoop stress is most compressive and exceeds the ultimate compressive strength (UCS) of the rock. In vertical wells, breakouts always form in the direction of the least principal horizontal stress. In deviated wells, the position of breakouts in a well is a function of the wellbore trajectory and the stresses acting on the wellbore. Therefore, when wellbore failures can be detected, their occurrence and characteristics (i.e., azimuth and width) can be used to determine in-situ stress magnitudes, effective rock strength, and stress orientations. In general, an abundance of stress-induced wellbore breakouts and some drilling-induced tensile fractures were detected. These fractures occurred intermittently throughout all three Unayzah formations. Cumulative length of these breakouts ranged from 86 to 142 ft in each well.