Limited entry, multiple injection matrix acidizing technology boosts well production in the world’s fourth largest gas reserves

March 1, 2014

Acid fracturing or matrix acidizing is often required for increased hydrocarbon production and long-term well deliverability from the massive Khuff carbonate gas reservoir in Saudi Arabia, the holder of the world’s fourth largest gas reserves.

Open hole multistage technologies have demonstrated superior performance in maximizing reservoir contact and productivity through better distribution of acid across the formation matrix, full interval matrix contribution and efficient propagation of fracture networks to bypass formation damage and optimize near wellbore conductivity.

The Khuff structure is a late Permian age heterogeneous carbonate sequence that underlies the massive Ghawar field in the Eastern Province of Saudi Arabia. The Khuff reservoir is subdivided into four separate intervals (A through D), with production coming mainly from the B and C intervals. Since its initial appraisal in the late 1970s, the majority of Khuff development has been focused in the relatively more prolific Khuff-C formation, where coiled tubing acid wash and single-stage acid treatments were repeatedly performed and evaluated. Over the past five years, multistage acid fracturing has been implemented in Saudi Arabia’s Khuff-C development. The results were carefully evaluated for each trial, and this is now the predominant Khuff-C stimulation technique.

Up until the middle of 2011, the vast majority of Saudi Arabia’s horizontal Khuff carbonate gas wells were drilled along the direction of maximum horizontal in situ stress (σmax). This was primarily to enhance wellbore stability and achieve the best possible penetration rates. Early multistage fracturing treatments in the Khuff generated mostly longitudinal fractures propagating parallel to the wellbore or in the σmax direction. Since then, a holistic approach toward the application of open hole multistage technology for tight reservoir development has been adopted.

The complex workflow of this approach calls for, among other requirements, changing the lateral section placement strategy and planning the horizontal section to be drilled along the minimum horizontal in situ stress (σmin) direction as opposed to the previous mode of planning along the σmax direction. Accordingly, understanding the reservoir stress profile, orienting the horizontal wellbore with respect to the dominant horizontal stress component and calibrating the stress profiles against actual open hole logs became the most important highlights of the new workflow. Radical improvement of stage integrity, multiple fracture signatures and enhanced well productivity were among the most important results achieved in developing the deep, tight gas-bearing zones of the Khuff carbonate reservoir.

Still, an innovative approach was required to address the more prolific zones of the Khuff-C formation where efficient matrix acidizing was sought as an alternative to acid fracturing in wells that could only be drilled in the σmax direction. Therefore, a purpose built open hole multistage technology system — one that was developed around the idea of distributed limited entry for placement of matrix acidizing treatments — was identified and carefully evaluated.

This article presents the details of the successful application of this new limited entry, multiple injection technology for optimized matrix acidizing of carbonate horizontal wells, including trial testing qualification, candidate selection, system design, functionality, operation and ultimate production profiling.

Tight gas, low permeability reservoirs present a tremendous challenge with respect to effectively completing and draining a target reservoir. Cased hole and open hole completions in horizontal wells offer a cost-effective means of accessing the entire lateral section, assuming the target pay can be effectively stimulated. While most open hole completions possess more advantages than cased hole completions, the challenge with open hole completions, compared to more conventional cased, cemented and perforated completions, is understanding and controlling the fracturing fluid flow into the near wellbore area of the reservoir.

The Khuff reservoir development has offered opportunities for a wide array of completion techniques to be implemented and evaluated, ranging from single stage vertical wells through multilateral wells to multistage horizontal wells. As the focus gradually shifted to tighter parts of the reservoir, the well completions underwent a process of increased complexity, from verticals to multilaterals and finally to horizontal multistage fracturing completions, which are also gaining popularity at the world scale as the industry taps more unconventional resources.

Early applications of open hole multistage completion technology in Saudi Arabia started in 2007 when the specified number of stages were run with a typical configuration of a single fracturing port between each two mechanical open hole isolation packers (1-5). Acid fracturing was conducted in the multiple stages through selective activation of the fracturing ports. It was observed that in wells drilled in the maximum horizontal in situ stress (σmax) direction, the first stage fracture will grow longitudinally, along the wellbore, parallel to σmax, causing the potential risk of overlap with subsequent induced fractures due to natural fractures and formation fissures. Initiation of the second and third fractures therefore became a challenge, due to possible pressure communication across the first induced fracture. To avoid this fracture overlap, it was decided that wells needed to be drilled in the minimum horizontal in situ stress (σmin) direction, allowing transverse fracture initiation perpendicular to the wellbore. The results from wells with open hole multistage completions showed increased initial production and less departure from the theoretical hyperbolic decline curves (6).

For wells designated for matrix acidizing stimulation, an issue rose regarding the ability of the single fracture port per stage to achieve uniform and effective stimulation of the entire stage length. To maximize the stimulated reservoir volume and reduce the likelihood of localized treatment of more prolific sections, it was necessary to think of a better way to address the specific needs for ensuring efficient stimulation and guaranteeing homogeneous distribution of the acid treatment across the stage length. This article discusses the first successful application of an innovative matrix acidizing technology in the more prolific zones of the Kuff carbonate gas reservoir.

There is often debate regarding the best approach for completing tight gas, low permeability reservoirs. This debate stems from the fact that tight gas reservoirs are more challenging to develop than traditional gas reservoirs and are relatively new in the industry due to previously existing technology and economic restrictions. One such debate involves the viability of the open hole packers with the fracture sleeves completion method to effectively stimulate long horizontal intervals of a targeted tight gas formation. The argument in favor of this completion approach is that a large area between the packers is exposed to the treatment fluid, providing the opportunity to create multiple fracture initiation points for the fracturing fluid and proppant to enter the target formation. This approach also has the advantage of being minimally influenced by near wellbore fluid friction constraints, such as perforation friction and/or perforation tunnel induced tortuosity, because the production casing or liner is not cemented. It is worth mentioning here that attempting to verify actual points of fracturing fluid entry and how many points of entry exist without advanced diagnostics is challenging, and fluid entry cannot effectively be modeled with conventional fracture modeling approaches.

The Permian Khuff-C carbonate formation is generally a prolific, nonassociated gas and condensate producing member of the giant Khuff reservoir in the Ghawar structure of Saudi Arabia. Extensive heterogeneity in stress profile, reservoir quality and reservoir fluids throughout the field, combined with the deep and extremely hot nature of the reservoir, makes uniform and effective stimulation of all layers a challenging task (7, 8). In this region, acid stimulation is required in the form of either matrix acidizing or acid fracturing to obtain high production rates and to add tie-in wells to the production facilities, all to meet the ever-growing demand for natural gas products. Matrix acidizing allows the removal of near wellbore damage induced during the drilling phase, while acid fracturing opens up channels beyond the near wellbore; both improve well productivity.

In matrix acidizing of prolific carbonate reservoirs, accurate acid placement is a major challenge as the acid tends to flow preferentially towards the highest permeability zones of the target interval (negative pressure effect), further increasing local permeability at these intervals and leaving the lower permeability regions of the formation untapped and untreated. To select the packer setting depths so that the packers are placed in competent formations and to refine the fracture stage interval lengths to target discrete pay intervals, the well’s local structure information and open hole log data should be carefully analyzed and reviewed.

In most cases, the open hole multistage tool layout is designed with uniform interval spacing. The spacing of packers and fracture sleeves is typically identical from stage to stage, with little regard to local geology and potential production units. For the design of this trial, the local structure information and open hole log data were used to position the packer setting depths so packers were in competent formations and to refine the fracture stage interval lengths and sizes to target discrete pay intervals. The intent of placing packers with varied spacing was to isolate fracture intervals with a similar log signature, which possibly indicates specific hydraulic flow units and discrete intervals with a propensity for production. This allowed the number of fracture intervals and stages, as well as the overall completion costs, to be optimized. Stimulation designs were prepared and tailored for each of the individual intervals.

It is also important to note that in this trial the geomechanical properties of the formation were predisposed to creating or tapping into a natural fracture network. Consequently, the formation had the potential to create a well-connected simulated reservoir volume, as opposed to discrete planar-type transverse hydraulic fractures. Therefore, a further goal of this particular trial test was to integrate that geomechanical information with the treatment data and the fully processed open hole logs. This resulted in more robust conclusions and recommendations for improving stimulation effectiveness when using this particular completion strategy.

The limited entry, multiple injection matrix acidizing assembly is best suited for matrix acid treatments in prolific and naturally fractured carbonate formations. Unlike standard open hole multistage completion systems (9-12), where there is only one fracture port per stage, the limited entry system features multiple jet nozzles placed in a single interval to create a strong matrix acidizing effect throughout the entire open hole interval (13), Fig. 1.

Stages are created using multiple shear-activated stimulation jets that are spaced out in the sections of interest and isolated by hydraulically set mechanical open hole packers, Fig. 2. The jet nozzles are adjusted and placed according to the reservoir characteristics determined from open hole logs, enabling controlled injection and leak off for effective flow of the acid treatment into the entire section of the interval. This effectively places the designed treatment at an optimal rate and pressure along the stage length, maximizing the development of complex conductive flow channels, also known as wormholes, throughout the entire stimulated reservoir length.

Each stage consists of a drillable cutter assembly pinned into a shear housing assembly. Downhole of the shear housing are shear-activated stimulation jet assemblies, spaced out with casing/ liner at predetermined depths. Above the lowermost packer in each stage is the locking/landing sub. The locking/landing sub provides isolation of this stage from lower stages and locks the drillable cutter to prevent it from rotating during milling operations. For effective setup of the system in the reservoir section, the liner and the annulus are isolated.

The liner isolation is achieved by the activation balls as they land on their respective seats in the cutter assembly, closing off the stages below. Multiple stages can be run, starting with the smallest activation ball and ending with the biggest ball size at the top. This mechanical diversion, combined with an advanced chemical diversion system, allows uniform, precise fluid placement.

Isolation of the annulus is achieved using open hole packers. The criteria for selecting a packer is to identify which packer will ensure efficient annular isolation between stages, cope with temperature cooling effects or the “shrinkage phenomenon” as cooler treatment fluids are pumped from the surface, and withstand high differential pressure cycles during fracturing so as to maintain the stability and pressure integrity of the completion system. Hydraulically set, mechanical, dual-element open hole packers are designed to withstand high differential pressures (up to 10,000 psi) during treatment cycles at reservoir temperature, Fig. 3a. Such dual-element packers provide the long-term isolation required to separate adjacent fractured intervals and so help ensure independent fracture propagation. These packers are equipped with a dynamic setting mechanism, which uses the elevated treatment pressures to continuously deliver additional pack off forces to the elements as the treatment pressures increase over the initial setting pressure inside the liner — a criterion that allows the packer to cope with the sudden downhole temperature decrease as colder treatment fluids are pumped from the surface (14).

Swellable packers, sometimes referred to as swellable element packers and/or reactive element packers, with swelling elastomer systems can also be combined with this assembly, yet they will remain passive, making no response to the dynamic temperature changes during pumping, Fig. 3b. Depending on the type of packer element, the design temperature and wellbore fluids, the elastomers can swell when exposed to the formation’s hydrocarbon or water.

In a limited entry, multiple injection matrix acidizing system, as acid is pumped from the surface, it is distributed evenly through the jets, where it interacts with the formation directly in front of the nozzles, Fig. 4. Therefore, it is prudent to place the jets in front of the sections that need to be treated first. The other sections of the reservoir will be treated as more and more acid is dispensed and spread along the wellbore in the open hole section.

The candidate well, Well-A, was a flank well drilled parallel to the σmax direction. After analysis of the open hole log, a threestage limited entry, multiple injection matrix acidizing system was specified with a hydraulic fracture port for the first stage and six limited entry stimulation jets for each of the second and third stages, Figs. 5a and 5b. The system was successfully deployed to total depth in the 4,000 ft thick gross pay, and matrix acidizing was pumped as per schedule for the three stages. As shown in Fig. 6, the opening and closing of each interval and the pumping of the acid treatment went as per design without any operational issues. Table 1 presents some of the main treatment parameters (15).

Figure 7 presents the wellbore layouts as well as the stress and porosity profiles of Well-A and three offset wells in the Khuff reservoir. The three offset wells — Well-B, Well-C and Well-D — are dual-lateral, deviated and vertical wells, respectively. The candidate well, Well-A, has a 1,300 ft net reservoir contact, laterally drilled in the σmax direction. Table 2 presents some of the reservoir and well characteristics as well as stimulation parameters for all wells. Each of the wells has been drilled and stimulated with different techniques; however, the reservoir flow capacity and permeability thickness product (kh) of the wells are comparable.

Open hole multistage technology has been implemented in various fields across Ghawar field to enhance productivity from moderate to tight reservoirs and to assess the technical and economic feasibility of this enabling technology in each field and each reservoir. Figure 8a highlights the distribution of open hole multistage technology applications in various deep gas development fields in Saudi Arabia. This spans carbonate and clastic reservoirs, and the treatments include both acid and proppant fracturing.

Figure 8b illustrates the sustained gas rate achieved from these different fields by applying open hole multistage technology. The total number of stages included in the evaluation is about 120. The number of fracture stages and the amount of acid or proppant pumped are dependent on reservoir properties and development. For the reservoirs that are currently being developed, five to eight treatment stages provide excellent coverage and production rates.

Most wells went through successful stimulation treatments as per design. The wells were subsequently cleaned up, flowed back, tested to confirm economic gas production rate and flowing pressure, and put on production. Figure 9 presents the normalized productivity index (PI) for the candidate Well-A and the three offset wells, showing the higher gas contribution from Well-A.

With the appropriate selection of candidate wells, treatments with both open hole multistage fracturing and limited entry, multiple injection matrix acidizing completions showed successful results, and each method contributed to high well productivity. Figure 10a presents the fold increase in well PI from the application of multistage fracturing over single-stage vertical fracture treatments. Figure 10b presents acidizing treatments where the application of limited entry, multiple injection matrix acidizing has superseded the standard multistage matrix acidizing.

The following conclusions are drawn from the work performed in the Khuff reservoir:

1. For this operation, the limited entry, multiple injection matrix acidizing technology components of stages 2 and 3 functioned successfully, as demonstrated by the pressure signatures of launching the cutter assemblies of these stages from the shear housings upon dropping the respective activation balls.

2. The limited entry, multiple injection matrix acidizing technology activation balls must be chased down the frac string with much higher pumping rates — 25 bbl/min to 35 bbl/min — as compared to the normal open hole multistage fracturing balls — 5 bbl/min to 7 bbl/min. The resultant higher fluid momentum creates a complex downhole hydraulic scenario, which usually alters the shape and profile of the pressure signatures on the pumping plot. This may not allow the various cutter releasing and landing steps to be captured on the plot and/or may actually translate into pressure responses on the surface.

3. The limited entry, multiple injection matrix acidizing technique:

• Evenly distributes the treatment across the interval by diverting acid to the entire isolated section of the open hole.

• Is a superior quality treatment over the conventional bullheading or coiled tubing acidizing.

• Is readily applicable to wells that have moderate to good reservoir permeability with reservoir heterogeneity and that require near wellbore stimulation and optimal acid dispersion along the treated interval.

• Is not a replacement for traditional multistage fracturing where discrete fractures and deep penetration are required, particularly in the moderate to tight gas reservoirs.

4. Both open hole multistage fracturing completion systems and the limited entry, multiple injection matrix acidizing technique have been successful in their own areas of application and have proven benefits when the proper well candidates are selected and treatment assemblies are successfully deployed.

The authors would like to thank the management of Saudi Aramco for their permission to publish this article. Also, special thanks go to the stimulation team at Saudi Aramco. Furthermore, the authors would like to express a very warm and sincere appreciation to Wael El-Mofty of Packers Plus Energy Services for providing valuable information with regard to the technology development and design. In addition, the authors are very thankful for the field operational crew and their continued dedication.

This article was presented at the SPE Kuwait Oil and Gas Show and Conference, Mishref, Kuwait, October 7-10, 2013.


σmax maximum horizontal in situ stress

σmin minimum horizontal in situ stress

kh permeability-thickness product, md-ft

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