Abstract

Acid fracturing treatments are used to enhance gas production from tight, deep carbonate reservoirs in Saudi Arabia.The produced gas has a regional variation between 0 and 10 mol% H[2]S. This variation in H[2]S content has impacted the metallurgy of well tubulars used in these areas.Gas wells in the high H[2]S area are completed with low-carbon steel (L-80 and C-95); those in the low H[2]S area are completed with super Cr-13 tubulars.

Acid systems used to fracture these wells included: regular, emulsified, and in-situ gelled acids, all based on 28 wt% HCl.However, 15 wt% HCl - 9 wt% formic acid was used to stimulate wells completed with super Cr-13 tubing. High temperatures (275ºF) encountered in deep wells, the presence of high H[2]S content and the use of large volumes of concentrated acids render corrosion control of tubulars a very difficult task.

Experiments were performed to develop a cost-effective acid system to enhance the productivity of deep gas wells, while maintaining the integrity of well tubulars.Acid fracturing treatments were monitored in the field and well flow back samples were collected following these treatments.The concentrations of key ions were measured in these samples including iron and manganese for completions with low-carbon steel tubulars and chrome, molybdenum, and nickel for completions with super Cr-13.

Analysis of flow back samples following the initial acid fracturing treatments showed that the return samples contained high concentrations of acid (up to 16 wt% HCl) and total iron (up to 20,000 mg/l). The presence of iron was a major concern during acid fracturing treatments. Modifications of the corrosion inhibitors package, increasing soaking time and over flush volume resulted in better results. Most importantly, the integrity of well tubulars was maintained. This paper discusses lab studies and application of these modifications in the field.

Introduction

Acid fracturing treatments have been conducted in a deep carbonate gas reservoirs.[1]Various acids are used in these treatments. Almost all of these acids are based on 28 wt% HCl. Corrosion control during these treatment has been a major concern. This is because of the use of concentrated acids, high temperature, and the presence of hydrogen sulfide in some areas in the gas reservoirs.

During pumping the acid down the tubing, the acid is continuously depleted from the active ingredients of corrosion inhibitors by the adsorption and/or polymerization on the metal surface. The active ingredients of corrosion inhibitor adsorb to the metal surface and form a thin film that protects the metal leaving the live acid with less inhibition. When the acid reaches the formation its inhibition is further reduced by adsorption on surfaces of rocks.Recovery of the spent acid back through the well tubing may cause corrosion problems if the return fluids contained live acid. This is mainly due to the low levels of corrosion inhibitor in the return fluids. In addition, surfactants, which are used to disperse the inhibitor in the acid, also have very high tendency to adsorb on rock surfaces.[2] This, in turn, leaves the return fluids with minimal dispersivity and poor functionality.

In general, cationic or partially cationic corrosion inhibitors adsorb to surface of the rock, especially in the case of sandstone. Nonionic corrosion inhibitors such as acetylenic alcohol show little if any adsorption on rock surfaces. Several authors have addressed the problem of corrosion inhibitor being adsorbed on formation rocks and thereby damaging the formation, altering wettabilty or causing emulsion problems.[3,4]

The concern of the presence of live acid in well flow backwas first raised by Huizinga and Like.[3] A similar work was done later by Morgenthaler et al.[4] Their experimental work was done on acidizing of sandstone rocks by HCl/HF acid. Simulated and real spent acids were used to test their effects on low-carbon steel (L-80) and stainless steel (super Cr-13). Both studies agree on that spent acid could be corrosive and may adversely affect the integrity of well tubulars. The extent of the problem can be addressed by a thorough understanding of the composition of the spent acid and its effect on various types of tubing.

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