Kristian Solhaug

Design Methodology for Swellable Elastomer Packers in Fracturing Operations

During past 7 years, swellable elastomer technology has been introduced to the oilfield, and its acceptance has been so rapid that its scope of application has rapidly expanded. Swellable technology employs the principle of elastomeric expansion in hydrocarbon or water to form a permanent seal. With the increasing acceptance of this concept for the oilfield, new applications are being explored, and this paper will discuss the application of swellable elastomer packers (SEPs) in stimulation operations. These applications include acid stimulations and/or hydraulic fracturing operations. During stimulation operations, SEPs are subjected to a very specific set of conditions that significantly affect the sealing performance of the tool(s). Three primary challenges must be addressed when designing SEPs for stimulation applications: 1. The downhole conditions; i.e., the main parameters to which the tool will be subjected such as the downhole pressure and the average temperature of the sealing elements. 2. Anchoring forces; i.e., the forces that occur from the shrinkage of the pipe during the treatment that subject the SEPs to additional forces that do not occur in conventional zonal-isolation applications. 3. Thermal contraction of the sealing element; i.e., the contraction that occurs due to the injection of the stimulation fluids that will cause the temperature of the packer to drop. This phenomenon causes the sealing element to partially contract and will result in altered sealing properties, and ultimately, the loss of the pressure seal. This paper describes the technical challenges and discusses resulting design methodology based on modeling (downhole parameters, anchoring forces) and laboratory testing (thermal contraction measurements) that have been developed to resolve these issues. This design methodology is not limited to stimulation applications but is applicable to any scenario with dynamic loads on SEP applications such as in water injection wells, etc. Finally, case histories are provided to illustrate the successes that have been achieved using the design methodology described.

Design Methodology for Swellable Elastomer Packers (SEPs) in Fracturing Operations

During past 7 years, swellable elastomer technology has been introduced to the oilfield, and its acceptance has been so rapid that its scope of application has rapidly expanded. Swellable technology employs the principle of elastomeric expansion in hydrocarbon or water to form a permanent seal. With the increasing acceptance of this concept for the oilfield, new applications are being explored, and this paper will discuss the application of swellable elastomer packers (SEPs) in stimulation operations. These applications include acid stimulations and/or hydraulic fracturing operations. During stimulation operations, SEPs are subjected to a very specific set of conditions that significantly affect the sealing performance of the tool(s). Three primary challenges must be addressed when designing SEPs for stimulation applications: 1. The downhole conditions; i.e., the main parameters to which the tool will be subjected such as the downhole pressure and the average temperature of the sealing elements. 2. Anchoring forces; i.e., the forces that occur from the shrinkage of the pipe during the treatment that subject the SEPs to additional forces that do not occur in conventional zonal-isolation applications. 3. Thermal contraction of the sealing element; i.e., the contraction that occurs due to the injection of the stimulation fluids that will cause the temperature of the packer to drop. This phenomenon causes the sealing element to partially contract and will result in altered sealing properties, and ultimately, the loss of the pressure seal. This paper describes the technical challenges and discusses resulting design methodology based on modeling (downhole parameters, anchoring forces) and laboratory testing (thermal contraction measurements) that have been developed to resolve these issues. This design methodology is not limited to stimulation applications but is applicable to any scenario with dynamic loads on SEP applications such as in water injection wells, etc. Finally, case histories are provided to illustrate the successes that have been achieved using the design methodology described. Introduction It has been approximately 7 years since swellable technology was introduced to the oil and gas industry, and since its introduction, many changes in the technology have taken place. Initially, the main application was the development of swellable elements for packers. These swellable elastomer packers were designed to provide zonal isolation in an openhole environment. As the technology became more and more accepted, and operators gained confidence in the capabilities of the technology itself and in running SEPs in wells, other applications were explored. The initial applications allowed operators to design wells with openhole completions, allowing for simpler operations (cementing and perforating were no longer needed) and higher production. Several other applications have emerged from the initial introduction: • Compartmentalization of the completion to prevent fines migration into the annulus • Cement assistance tools (CATs) that were developed with elastomer technology to fill in portions of the incomplete cement sheath and fill the gaps resulting from the casing2 RUTGER EVERS, DUSTIN YOUNG, GREG VARGUS and KRISTIAN SOLHAUG SPE 116256 cement de-bonding phenomenon. The combination of hydrated cement and the cement assurance tool provides increased assurance that high-quality zonal isolation will result • Low-cost alternatives for the low-pressure market • Application of swellables in multi-lateral (MLT) wells • Straddling off water and / or gas zones using drop off tools. SEPs offer significant operational benefits to the operators in addition to robust and reliable, long-term zonal isolation because of: • Operational simplicity, since no personnel or setting tools are required (See Figure 1) • Extremely low failure rate, since no mechanical parts are in the tool • High sealing efficiency, since the elements will conform to any irregular shaped areas along long openhole sections. The above advantages have enabled operators to revisit openhole completion scenarios. It is for these reasons that SEPs have become the preferred solution over mechanically or hydraulically set packers for many operators for stimulation treatments. Mechanically set packers do not always provide a sufficient hydraulic seal in open holes, and the capabilities of the SEPs in this regard enable operators to consider the benefits of the openhole environment. At present, over 10,000 installations have taken place worldwide in almost 50 countries for small independent operators, national oil companies (NOCs), and international oil companies (IOCs). Hydraulic Fracturing and SEPs With the growth of SEPs for stimulation treatments, additional research and development efforts have been devoted to investigation for other applications. One of the areas considered has been hydraulic fracturing, and by combining multidisciplined fields of expertise, reliable designs have been developed to support this application. The use of SEPs in hydraulic fracturing mainly focuses on the North American market at present, but other geographical areas can benefit from the results of the design methodology developed. All knowledge and expertise learned and documented for this application can be used for any application where a cold fluid is pumped down the tubing, resulting in a cool-down effect in the element from the tubing side. The objective(s) of a hydraulic fracturing treatment are: • To bypass near-wellbore damage to re-establish natural productivity • To extend a conductive path into a formation to increase productivity beyond the natural level • To alter flow in the formation. To achieve the fracturing objective, a sand or proppant-laden fluid is pumped downhole. Fractures in the formation are created by pumping the fluid into the formation above the formation fracture gradient (FFG). This means that the fluid is pumped into the well faster than the fluid can escape into the formation. Pressure rises, and at some point, the formation will break. The breakdown and early fracture growth expose new formation area to the injected fluid: The rate of fluid loss increases, but as long as pump rates are maintained higher than the fluid-loss rate, the newly created fracture will continue to propagate and grow. Once the pumping stops, and the injected fluid leaks off, the fracture will close and the new formation area will not be available for production. To prevent this from happening, a proppant (sand) is added to the hydraulic fluid to be transported into the fracture. When pumping stops, and fluid flows back from the well, the sand remains in place to keep the fracture open and maintain a conductive flow path for the increased formation flow area during production. Pump rates and volumes of fluids pumped are usually high and can be anywhere from 40 to 90 bbl/min. Due to the high rates, the fluids will not heat up much while being pumped downhole, resulting in a cooling effect of the fluid from the tubing side. The temperature drop mentioned above has two major effects on the packer; thermal contraction of the swellable element and thermal contraction of the tubing will occur; these changes result in pulling forces on the packer. Both effects should be modeled and quantified to enable the SEPs to be designed appropriately for the application. The above mentioned scenario with the dynamic load on the SEP elements does not appear during hydraulic fracturing only; injection wells and other stimulation operations such as acid treatment and gravel packing where the fluid pumped will result in a cool down are also affected. Therefore, both the design of the SEP and the job execution for hydraulic fracturing will require the combination of several technologies and competencies. Design methodology This section explains the design process step by step. The critical design parameters are identified, and their impact on the SEP performance is explained. Describing downhole conditions. As is the case with any model, the results rely heavily on the input that is used. Two parameters play a major role in the design of any SEP, and for applications in a stimulation operation, temperature and pressure are particularly important. SPE 116256 DESIGN METHODOLOGY FOR SWELLABLE ELASTOMER PACKERS IN FRACTURING OPERATIONS 3