Well Stimulation Technology for Thick, Middle East Carbonate Reservoirs
Authors: Abou-Sayed Ibrahim , Shuchart Chris E. , Choi Nancy Hyangsil , Clancey Byron Michael , Bene Thomas Franklin 
Published: February 2016

Previously, ExxonMobil had undertaken a multi-disciplinary approach to develop and integrate the required technologies for design, implementation, and evaluation of acid treatments in thick heterogeneous carbonate reservoirs.1 RasGas, in collaboration with ExxonMobil, has customized the technologies and integrated methodology for application in a major field in the Middle East with a high level of success.

Acid placement and diversion are critical to achieving effective stimulation in heterogeneous carbonate reservoirs. While permeability is a major factor in the distribution of acid along a completion for many reservoirs, pre-stimulation skin damage, intermixed rock types with different acid-rock wormholing characteristics, distance between zones, and differential reservoir depletion also play important roles in the effective stimulation of the reservoirs.

Important steps in the integrated methodology developed and implemented for matrix acidizing include:determine the stimulation requirements given the well/reservoir objectives,characterize the various rock types present in the formation,develop an integrated perforation/stimulation strategy,conduct appropriate laboratory tests with representative field core plugs,model the stimulation process with tools calibrated to the formation of interest,develop field procedures and implement the treatments as per design,evaluate stimulation effectiveness, andoptimize treatments based on post-stimulation performance and operational constraints.

This paper features some of the technologies that have been developed and describes the integrated methodology used to effectively stimulate thick carbonate reservoirs in the Middle East.

Introduction

The technology of carbonate matrix stimulation has advanced significantly over the past 10 years through innovative laboratory testing, new fluid developments, and advanced computer models to simulate the process. However, the existing approaches are not sufficient to meet the challenges of optimized stimulation of wells in massive carbonate reservoirs. Typically, the intervals to be produced in these reservoirs are very thick and highly heterogeneous. The permeability can range from a few milliDarcies to several Darcies.

ExxonMobil and RasGas have jointly developed an integrated methodology to optimize matrix stimulation of the Khuff reservoir, a large, complex carbonate reservoir in the North Field of Qatar. The integrated methodology is a continuous process which consists of five main elements: reservoir objectives, completion strategy, stimulation design, implementation, and evaluation. The process was introduced by ExxonMobil in an earlier paper, which focused on the general laboratory testing and process modeling.1

This paper discusses how the integrated carbonate stimulation methodology was customized and implemented by RasGas and ExxonMobil as a critical component of the North Field development, focusing on the stimulation strategy, design, and results obtained. The carbonate stimulation design methodology cycle, specifically as applied to the North Field, is shown in Figure 1.

The reservoir objectives for the development of large fields often span multiple heterogeneous producing horizons containing many layers with varying rock properties. From a resource standpoint, the ultimate objective is to economically extract the maximum amount of hydrocarbon from the reservoir. In order to accomplish this, the optimum production flow profile for reservoir depletion is required.