Evaluation of Interaction of Acid Systems and Formation Samples Improves Production Response
Authors: Metcalf A.S. , Devine C.S. 
Published: March 2010

Economic stimulation of carbonate rocks with acid systems is one of the most efficient means of increasing production; however, to accomplish success, an understanding of the interaction between proposed acid systems and the formation mineralogy is paramount, not only from a chemical reaction perspective but also from a physical interaction perspective. Understanding these interactions is of even greater significance with increased reservoir temperatures.

Successful stimulation starts with an understanding of the reservoir pore system and composition. An evaluation of fluids to achieve the best differential etching and maintain adequate rock integrity to withstand closure of created conductivity paths is essential to achieving long-term improved production. Obtaining paths of economical penetration is accomplished through the control of both leak-off and reactivity.

Case histories covering a wide range of reservoir conditions and geographical areas are presented in which the above mentioned evaluations were implemented to verify or support changes in treatments to improve productivity. Production responses varied from a 20% gain to a 500% gain over other typical treatments in offset wells.

Introduction

Acidization has long been utilized as a stimulation technique for carbonate reservoirs. Early acid consisted, frequently, of small volume acid dumps down the wellbore with the hope of production improvement in the near wellbore region through the dissolution of near wellbore rock. Little attention was given to the rock type (limestone, dolostone, a mixed carbonate, shaley, sandy, or anhydritic) and pore system morphology, to the "hardness" or "softness" of the carbonate, and virtually no attention was given to the manner in which acid reacted with the rock, either relative to etch patterns or to rates of reaction.

Subsequently, and with improvements in pumping equipment and the development of inhibitors, the acidization of carbonates could occur with higher volumes of acid and higher pump rates. Quasi reaction rates were determined by static rate testing, and success was assumed, relative to the generation of a conductive fracture. Treatments did occasionally fail, and workers began examining the actual rock relative to the rock composition, the rock hardness, etching patterns developed on rock surfaces, and dynamic rates of dissolution of the carbonate in acids of various strengths.

For the past several years, considerable attention has been given to the improvement of carbonate acidizing and acid fracturing by determining applicable rock properties (composition, strength, hardness, the pore system morphology), by determining acid reaction rates specifically for the reservoir of interest, and by determining etching patterns(1–4). In this paper, detailed laboratory studies and corresponding field results are discussed. Where possible, comparisons are made with previous acid treatments.

Experimental

Laboratory testing was initiated to define rock properties, pre-and post-acidization Brinell hardnesses, and acid reaction rates for use in acid stimulation design for field application.

Rock Mechanical Properties

Cylindrical core plugs, 2.54 cm in diameter and 2.54 to 5.08 cm in length, are drilled from whole core sections, using Isopar L oil or KCl water. Plug ends are ground flat and parallel to within a tolerance of 0.025 mm. Samples are left in the as received condition.