A New Approach to Matrix Sandstone Acidizing Using a Single-Step HF System: A Niger Delta Case Study

Abstract

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractThe use of unique or modified HF acid systems for matrix acidizing within the Niger Delta and some other parts of the world is still the preferred method for effective stimulation of sandstone reservoirs. The continued success of treatments done with modified HF systems in this area are due to its ability to mitigate the rapid spending of the active acid with clays and silicates; prevent matrix unconsolidation in the near wellbore region and the subsequent precipitation of acid reaction by-products within specific well applications. This success has brought about modifications to the acid system and new applications for various well scenarios. This paper discusses the successful application of a unique sandstone acidizing process that uses a single acid stage (no preflush or postflush is required) making multiple stage treatments much simpler while still achieving both economic and technical targets for the well. It covers the non-formation stimulation treatment of several wells using this single-step method and also describes a successful approach to improving the productivity of high water cut wells with severe finesdamaged gravel packs by this system. Unique experiences acquired during the field trials carried out within the Niger Delta using this system are evaluated. The results demonstrate a significant increase in incremental hydrocarbon production for normally high risk, low reward wells and remarkable stimulation cost reduction via reduced chemical volumes, less equipment requirements and shorter job times.Also included are a thorough analysis of actual candidate selection criteria, fluid chemistry, actual job design and operational issues during execution of treatments. Key technical and economical performance indicators including skin factors, production rates, specific productivity indices, treatment costs indicate that this system has certainly and successfully increased the application envelope for unique HF treatments within sandstone reservoirs.

Automating sandstone acidizing using a rule-based system

“…The HV acid reacts with ammonium bifluoride (NH 4 HF 2 ) or with ammonium fluoride to produce HF acid. In order to produce a 1 % HF acid solution, 20 gal of HV acid per 1,000 gal of water are required to react with approximately 123 lbm of NH 4 HF 2 (Uchendu et al 2006). This self-generating reaction of HF acid reduces the rate at which the acid system reacts, and therefore, allows an increased depth of penetration of live HF acid into the formation.…”

Experimental study of one-step economical retarded sandstone acid

“…To overcome the problems of conventional mud acid in high-temperature sandstone reservoirs, many attempts have been made from acid systems (Li et al 2015;Shafiq et al 2013;Zhou 2013;Mahmoud et al 2011;Thomas et al 2001;Gdanski 1985). One of these attempts was to develop retarded mud acid systems.…”

“…However, at elevated temperatures, compared with the conventional mud acids, these retarded mud acid systems still have some problems, such as new precipitations produced. For example, in the H 3 BO 3 system, Thomas et al (Thomas et al 2001) pointed out that when H 3 BO 3 reacts with feldspar, KBF 4 precipitates are formed. In the AlCl 3 system (Gdanski 1985), first AlCl 3 reacts with HF to form AlF 4 and then as HF is consumed, AlF 4 is hydrolyzed to HF.…”

“…At a temperature of up to 149 °C (300°F), compared with conventional mud acid, phosphonic retarded mud acid was used to increase the core permeability by 177.9% (Shafiq et al 2013). Many studies were conducted on various acid combinations as an alternative to conventional mud acid, mainly focusing on the combinations of HF:H 3 PO 4 and HBF 4 : HCOOH (Shafiq et al 2017;Shafiq et al 2013;Thomas et al 2001). The experimental results indicated that the best acid combination of 3%HF:9%H 3 PO 4 with a permeability increase of 135.32% is better than the conventional mud acid with a permeability increase of 101.76%.…”

“…A few studies have been reported on different sandstone reservoirs at elevated temperatures. And the current acid combinations still have some limitations at high temperatures such as fast acid-rock reaction rate, short acid penetration distance, instability of clays and pipeline corrosion (Abaa et al 2017;Barri et al 2016;Gomaa et al 2013;Thomas et al 2001). Thus, it is very significant to develop economical acid combinations, which can minimize the limitations at high temperatures.…”