Optimization of Carbonate Stimulation based on Long-Term Well Performance Predictions
Authors: Postl Dieter , Ellison Timothy Kirk , Chang Dar-Lon , Shuchart Chris E. , Mols Arnout Laurens , Nor Nazri , Al-Kharaz Hani Abdul Redha , Valle Antonio , Sieben Christopher John , Chintaluri S Ram , Wang Zhihua , Sanchez Luis , Farah Ali M. 
Published: March 2015

Individual wells and their respective completions in the North Field of Qatar are becoming increasingly important and sophisticated. Effective matrix acid stimulation of the very thick, layered carbonate reservoirs plays a key role in achieving and maintaining production targets to meet present and future demand. Completion intervals routinely traverse several layered reservoirs, commingle multiple zones, and span more than a thousand feet in length. The very long intervals give rise to large differences in hydrostatic pressure during stimulation treatments, further complicated by differential reservoir pressures and rock properties (permeability, porosity, acid reactivity) that change both vertically and areally. In addition to this reservoir complexity, multiple stimulation approaches developed for these high-rate gas wells introduced additional variables in the stimulation design. These included mechanical isolation, chemical diversion, and increased treating rates, all of which carry varying degrees of benefits, costs and risks.

Close collaboration between RasGas Company Limited (RasGas) and ExxonMobil has produced a comprehensive toolkit and methodology to effectively stimulate each well and maximize its long-term performance at substantially lower cost and reduced operational risk. The methodology provides an integrated process for quantitatively evaluating and comparing stimulation options in terms of production performance, optimizing these options in reference to physics-based performance limits, and selecting options considering cost, operational complexity, risk, and reward.

The theoretical limits discussed in this paper provide a powerful reference capability that can bracket expected performance. A key learning from the study is that stimulation designs can be implemented that allow wells to perform near their "physics limit" at significantly reduced cost and considerably lower operational risk than previously understood.

Introduction

The North Field is the largest non-associated gas field in the world with estimated reserves exceeding 900 TCF. Qatar's North Field energy resources and RasGas' role in extracting and distributing those resources have significant impact on the global LNG market. Industry experts estimate that Qatar will supply 25–30 per cent of the world's LNG by 2010. RasGas will produce, process, and ship almost one-half of Qatar's LNG. As part of this rapid expansion of the North Field development, RasGas has drilled and completed nearly 100 wells over the past five years.

The main completions objective is to construct long-lived wells that maximize wellbore deliverability at reduced cost, while increasing reliability and decreasing operational risk. To achieve this objective in the North Field, effective matrix acid stimulation is of critical importance. Completion intervals in RasGas wells routinely traverse several layered reservoirs, commingle multiple zones, and span more than a thousand feet in length. The very long intervals give rise to large differences in hydrostatic pressure during stimulation treatments, further complicated by differential reservoir pressures and rock properties that change both vertically and areally (e.g., permeability, porosity, acid reactivity, etc.).