Use of Weak-Acid and Nonacid Alternatives in Acidizing Horizontal Wells

Abstract

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 171766, “Challenge of Acidizing Horizontal Wells in Tight Carbonate Reservoirs: Weak-Acid and Nonacid Alternatives,” by Sameer Punnapala, SPE, Maryam Abdul Rahman, SPE, and Sanjay Misra, SPE, Abu Dhabi Company for Onshore Oil Operations, prepared for the 2014 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 10–13 November. The paper has not been peer reviewed. An operator developing tight carbonate reservoirs found that conventional acid stimulation with 15% hydrochloric acid (HCl) in horizontal wells did not provide expected results. A subsequent study revealed that permeability and length of treatment are the main parameters in deciding the optimal stimulation chemical. A correlation was developed, on the basis of the kinetic parameters of various chemicals and field experience, that can act as a guide in selecting the proper acidization formulations. Introduction In vertical wells, positive skin pertains to reduction of effective wellbore radius, thus sacrificing reservoir contact. It is very important to note that, in long horizontal wells, wellbore radius does not represent the reservoir contact; rather, it is replaced by the horizontal length. It can be said that the horizontal well itself represents a limiting case of the finite-conductivity fracture. Thus, in the case of a homogeneous reservoir, if vertical permeability is sufficient, the horizontal well is supposed to drain the reservoir. The primary aim of stimulation in such cases becomes removal of near-wellbore formation damage. The field development of this operator’s tight carbonate reservoirs was envisaged as being performed by drilling 3,000- to 5,000-ft-long horizontal wells. Although 15% HCl is the conventional acidization formulation for these fields, a study of conventional acidization jobs carried out in these fields revealed that HCl stimulation did not yield the desired results. Selective stimulation in a segmented completion and by use of diversion also did not change the situation. The operator therefore launched an extensive market and literature survey to conduct a root-cause analysis. The study revealed that weakacid chemicals and nonacid chemicals (e.g., chelates and nanoemulsions) offer a viable alternative. On the basis of the study, using kinetic and transport parameters of different chemical and coreflood studies cited in the literature, a correlation was developed for formation permeability and length of treatment vs. effectiveness of different stimulation recipes. This correlation was validated by the results obtained from the acidization review, and a reasonable matching was obtained.