Effective Stimulation of Very-Thick Layered Carbonate Reservoirs Without Use of Mechanical Isolation
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 13621, ’Effective Stimulation of Very Thick, Layered Carbonate Reservoirs Without the Use of Mechanical Isolation,’ by C.E. Shuchart, SPE, S.K. Jackson, SPE, J. Mendez-Santiago, N.H. Choi, SPE, J.K. Montgomery, SPE, and A.S.D. Khemakhem, SPE, ExxonMobil Up-stream Research Company, and C.J. Sieben, B.M. Clancey, SPE, R. Chintaluri, SPE, A. Farah, SPE, and Z. Wang, SPE, RasGas Company, originally prepared for the 2009 International Petroleum Technology Conference, Doha, Qatar, 7-9 December.

Effective matrix acid stimulation is one of the keys to maximizing and maintaining long-term North Field well productivity. The operators jointly developed an integrated methodology to optimize matrix stimulation for layered Khuff reservoirs, specifically for K1 through K3 and K4 completions. The integrated methodology is a continuous process to help overcome the well and reservoir challenges. 

Introduction

Completions are the critical component of a well that connect the wellbore to the reservoir. Despite the prolific nature of the North Field reservoir, optimized completions are necessary to ensure long-term well deliverability. As in many developments, reservoir quality (i.e., permeability thickness and connectivity) and completion efficiency have the greatest effect on well productivity. Effective completions/stimulations have a positive effect on long-term deliverability by minimizing completion pressure drop throughout project life. By wormholes, acid stimulation also can help mitigate future productivity-impairment mechanisms, such as condensate dropout, non-Darcy flow, and scale/solids plugging.

ExxonMobil developed an integrated methodology to achieve optimum matrix stimulation of large, complex carbonate reservoirs. The integrated methodology for the stimulation of thick carbonate reservoirs is a continuous process that consists of five main elements: reservoir objectives, completion strategy, stimulation design, implementation, and evaluation (Fig. 1). Building upon this foundation, ExxonMobil and RasGas jointly developed an integrated methodology to optimize matrix stimulation for separate sections of the Khuff reservoir (K1 through K3 and K4).

In addition to the many wells, the K1-through-K4 development wells required longer completions through all four Khuff reservoirs, with completion lengths as great as 2,500 ft. Success of K1-through-K3 completions and individual K4 completions led to high expectations for commingled K1-through-K4 completions. However, the longer K1-through-K4 producing interval substantially increased the stimulation challenges. Additionally, differential pressures between reservoirs had developed during production in various parts of the field. This further challenged acid placement and created potential for crossflow once the well was stimulated. Existing stimulation tools and methods were no longer sufficient to achieve the aggressive stimulation targets required to meet completion objectives. Development of alternative K1-through-K4 completion/stimulation strategies was required.