Summary Acid-fracturing treatments are used commonly to enhance the productivity of carbonate formations with low-permeability zones. Various forms of hydrochloric acid (HCL) are used to create deep etched fractures. However, regular HCl reacts very fast with limestone and high-temperature dolomite formations and, unless retarded, will produce a fracture with low conductivity. In addition, concentrated HCl-based acids are very corrosive to well tubulars, especially at high temperatures. To address problems associated with concentrated acids, various retarded acids were introduced. Organic acids were used also in some cases. These organic acid systems were used successfully to acid fracture several wells in a deep gas reservoir in Saudi Arabia. Field data, however, indicated that there is a need to create deeper and more-conductive fractures. To achieve this goal, it was decided to conduct a field trial with a newly developed acid system. The new acid system is an ester of an organic acid in the form of solid beads. The ester reacts with water (hydrolyzes) at bottomhole temperature and produces lactic acid, which reacts with carbonate minerals and etches the surface of the fracture. The system was examined thoroughly in the laboratory and showed promising results. The treatment was conducted in the field without encountering operational problems. After successful placement of the solid beads in the fracture, the well was shut in for 24 hours to give ample time for the ester to hydrolyze and for the generated acid to react with the formation rock. The well was allowed to flow, and samples of the fluids produced were collected to understand chemical reactions that occurred during the treatment. The treatment has resulted in a slight increase in gas production, and no significant improvement was noted over a 9-month period. Consequently, the well was matrix acidized with 28 wt% HCl and responded positively to the treatment. This paper will discuss major reactions that occurred during these treatments and how they impacted well response. Lessons learned and recommendations to improve the results of this new acid system will be given.
Saudi Aramco has significantly increased the number of horizontal gas producers over the past five years in an attempt to maximize well productivity through maximum reservoir contact while reducing the need for hydraulic fracturing stimulation, commonly required in most vertical producers. Although the majority of horizontal producers have met expectations, a good number of them have underperformed showing lower initial gas rates and higher decline rates than anticipated. The unexpected poor performance in most wells has been attributed to induced formation damage during drilling operations, the highly heterogeneous nature of the reservoir resulting in low porosity and permeability in different areas of the field, destabilization and precipitation of mineral and organic scales in the tubulars and formation, and condensate banking effects. The overwhelming majority of horizontal producers are completed open-hole, which significantly limits stimulation options once the wells have been completed and put in production. Moreover, high pressure and temperature conditions make accessibility of the horizontal section very difficult, and the chance for achieving successful stimulation of the entire horizontal section relatively low. Conventional stimulation treatments involving coiled tubing (CT) acid washes and bullheaded hydraulic fracture treatments have been attempted over the years but results have been mostly unsuccessful or relatively modest. This paper details a case history of a new approach combining different techniques, which was successfully used to stimulate a long open-hole horizontal section in an underperforming gas producer. A hydraulic fracture treatment using hydrajetting technology, that provided the ability to achieve deep penetration into the targeted zone, resulting in effective treatment fluids diversion, was successfully pumped. A detailed analysis to ascertain the effectiveness of the procedure was conducted, the results of which are discussed in this paper. Introduction Gas producer A1 is located in a partially depleted sector of the field where reservoir characteristics are those of a tight formation. The well's reservoir pressure averages 5,000 psig. at 10,000 ft TVDSS, and it has a kh of 0.4 md-ft, which partially explains the reason the well produced at a low average pre-stimulation rate of 2 MMSCFD with a flowing wellhead pressure (FWHP) of 1,200 psig. Moreover, the well is flowing at below dew-point pressure, thus condensate banking and mineral and organic scale deposition effects were additional contributors to the well's poor performance. In fact, condensate banking and scale deposition are challenges currently facing a number of gas producers that have been on-stream for a number of years, and Saudi Aramco has a number of ongoing research projects and scheduled field trials aimed at addressing and solving these problems.
Summary Thick deposits of various types of mineral scales are presently forming in the tubulars and formation of gas producers drilled in Saudi Arabian carbonate reservoirs. These mineral scales precipitate when ideal thermodynamic conditions combine with dissolved minerals present in formation waters. Without remedial action over time, these deposits can grow thicker and end up plugging tubulars and the reservoir. Thick deposits of mineral scales have recently begun to appear in gas producers in certain areas of the field. A comprehensive study was conducted to ascertain the nature of the precipitation mechanism and identify potential solutions to the problem. This paper details how laboratory analysis data, well production history, reservoir geology and petrophysics, and reservoir description were analyzed and used with sophisticated computer software to identify the formation-damage mechanism and the different scale types precipitating in the wellbore and formation. Extensive simulation work was conducted as part of the study to forecast the type and amount of mineral-scale precipitation that can be anticipated at varying reservoir and producing conditions. The study also evaluated the most cost-effective and feasible ways to remove different types of scale deposits. The future scale-inhibition and -removal strategies to be implemented in existing and future gas producers are being derived in large part from the results of the study described in this paper.
One of the key strategies in Saudi Aramco's optimum gas development project is drilling single and multilateral wells to achieve maximum reservoir contact to maximize well productivity. This strategy has proven very successful over the past few years as the majority of the horizontal gas producers have yielded excellent results, with open-hole completions in particular. Consequently, most of the planned future wells will be drilled as open-hole horizontal completions. Nonetheless, due to the highly complex nature of the Khuff carbonate reservoir some wells have experienced complications during the drilling phase and encountered unexpected reservoir challenges which has kept them from achieving their production targets. These wells require stimulation to regain their productivity, but the available choices to achieve effective stimulation in horizontal open hole completions have traditionally been limited at best. Previous stimulation attempts with coiled tubing have yielded modest improvements mainly due to difficulty in accessing the long, deep and high pressure/temperature zones requiring treatment. Bullhead treatments at high injection rates have also yielded similar modest well productivity enhancements, because of the inability to achieve effective diversion. Consequently, Saudi Aramco field tested an innovative new completion technology that provided the ability to selectively fracture stimulate multiple zones along the horizontal section. Field trials have yielded excellent and highly promising results, which compared favorably with the performance of non-stimulated offset wells. It is well-known in the industry that fracture stimulating horizontal open-hole completions with conventional methods is a challenging endeavor, and that often times results are disappointing. Nevertheless, the completion technology tested by Saudi Aramco proved capable of overcoming most of the challenges associated with horizontal fracturing, and effective stimulation of the long horizontal section was achieved. Through this technology multiple mechanical isolation points are created in the wellbore using specially designed multiple open-hole packers, and then selected zones of varying lengths can be individually treated in accordance with reservoir characteristics and production targets. This paper details the planning and design processes leading to the implementation of the first field trial, experience and lessons learned during deployment of the completion and implementation of the stimulation treatment, and the post stimulation results. Suggested optimization steps from this successful experience are discussed as a way to further enhance the benefits from the technology in future applications.
TX 75083-3836, U.S.A., fax +1-972-952-9435. AbstractAcid Fracturing has been a successful method to stimulate the Khuff Carbonate wells of Saudi Arabia since the beginning of the gas development program. Various types of acid systems including conventional, emulsified and surfactant-based have been used in an attempt to achieve optimum fracture length and conductivity. Acids used for these treatments have been typically formulated with 28-wt% HCl and have been used successfully to increase production from the Khuff carbonates.Although acid fracture treatments create significant conductivity enhancement in treated wells, their etched fracture length is typically short because of the high speed at which acid spends upon contact with the high temperature reservoir. The quest to increase the effective half-length of the fracture and enhance production led to the search for novel effective technologies capable of achieving this goal. One such technology is the solid acid system which was field tested for the first time in the world in a Saudi Aramco gas producer. The material is pumped as an inert solid, which hydrolyzes and converts to acid when exposed to water and heat resulting in heterogeneous etching and increased conductivity. Among the key factors supporting a field trial of the product was the fact that it offered the prospect of overcoming the traditional challenges that are always associated with acid fracturing such as excessive acid leak-off, shallow acid penetration, fast spending due to the temperature effect and corrosion. However, a solid acid system also reduces the health, safety and environmental (HSE) risk caused by conventional acid exposure to both personnel and production equipment. Placement of this material in the Khuff was a significant challenge due to the high fracture gradients encountered in this complex reservoir. Previous attempts to place proppant in other wells with similar reservoir characteristics resulted in premature screenout. Nevertheless, the first worldwide field trial of the new technology was successfully implemented. This paper will discuss in detail the design, execution, post-stimulation and the long term results from this first trial. It will also provide conclusions and recommendations on the effectiveness of this novel technology and suggest areas of improvement that will help in promoting a step change in acid fracturing industry worldwide. 2 IPTC 12668
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