An Overview of Frac Packs: A Technical Revolution (Evolution) Process

Abstract

Technology Today Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering. Introduction Several completion methods are the current options typically considered when completing oil and gas wells in unconsolidated sand reservoirs. These include(1) rate control, where the flow rate is held below the sand-production threshold or below erosional limits handling produced sand at surface; (2)screen-only completions, which are common to horizontal and extended-reachwells; (3) underreamed openhole gravel packs; (4) cased-hole gravel packs; (5)in-situ resin treatments; (6) high-rate water packs; (7) water fractures; (8)" frac packs" ; and (9) screenless frac packs. Frac packing is a developings and-control alternative, and use of this method continues to increase in both absolute numbers and percentage of all sand-control completions implemented. Frac-pack pumping strategies represent a definitive break in the long history of sand-control developments. Most early procedures categorically avoided pumping strategies that "break down" unconsolidated formations. Discarding the" don't-break-down" paradigm has resulted in a new generation of sand-control concepts. This paper provides a brief overview of frac-pack evolution and discusses how growing acceptance of the method is creating a revolution in well-completion practices where sand control is required. Frac-Pack Beginning Focus on frac-pack developments started about 1987 after convincing technical and economic achievements resulted from application of tip-screenout(TSO) hydraulic-fracture stimulations in North Sea chalk formations and in high-permeability Prudhoe Bay and Kuparuk field formations on the North Slope in Alaska. The TSO-fracture method significantly increases proppant loading in a fracture, providing substantial conductivity improvement. High fracture conductivity resulted in dramatic and sustained productivity improvements and prompted application of TSO-fracture stimulation in high-permeability, unconsolidated formations. Amoco experimented with completions in 1964 in the Hackberry field, Louisiana, that included fractures in conjunction with sand-control completions(call "hack fracs"). The term frac pack was actually used as early as the late1950's by Shell in Germany for sand-control completions that were fractured before gravel packing. Current use of frac pack refers to completions that combine TSO hydraulic-fracture stimulation pumped with a gravel-packscreen packer assembly in place. The TSO fracture creates a short, highly conductive fracture that is designed to bypass near-wellbore damage. The gravel-pack screen assembly prevents proppant flowback. The proppant/formation interface establishes a stable barrier, preventing sand production. This increase in effective wellbore radius reduces (and often eliminates) positive skin and mitigates factors related to fines-migration problems (see Figs. 1through 3). Frac-pack case histories from the Gulf of Mexico, Europe, Africa, South America, and the Asia-Pacific report increased productivity and effective sand control. In the past 10 years, the use of frac-pack completions has steadily increased in both absolute numbers and as a percentage of sand-control completion methods deployed throughout the world. Significant investment by the service sector has increased the number of purpose-built fracture boats for offshore operations in the past 4 years. Further expansion of these resources is under way. Frac packs are often considered whenever and wherever sandcontrol is required. During the early frac-pack development period, operator and service company innovators often discussed minifracture interpretation methods, design strategies, implementation techniques, and job procedures. Sharing of experiences, observations, concerns, and nonproprietary results was fairly widespread. This open dialogue significantly accelerated frac-pack evolution. In 1992 and 1993, SPE chapters in Houston and Lafayette, Louisiana, sponsored meetings and study-group sessions to present frac-pack developments and address opportunities to improve frac-pack techniques and procedures. The SPE Gulf Coast Section organized a 1-day frac-pack workshop in 1994 and has held three more since. In 1996, "Frac Packs and Other Sand Control Methods" was an SPE Forum Series in North America topic. All these technology exchange sessions had excellent industry participation. Early Concerns At the beginning of the frac-pack revolution, application of TSO-fracture stimulation to highly deviated, typical Gulf of Mexico wells was a controversial concept and many concerns were voiced. The most frequently stated issues were increased cost, inadequate gravel-pack tool capabilities, fear of fracture-height growth into nearby water sands, and lack of coincidence between a vertical fracture and a deviated wellbore.