Stimulating Unconventional Reservoirs: Lessons Learned, Successful Practices, Areas for Improvement

Abstract

The term "unconventional reservoir" has different meanings to different people. Certain reservoirs termed unconventional have a rock matrix consisting of inter-particle pore networks with very small pore connections imparting very poor fluid-flow characteristics. Abundant volumes of oil or gas can be stored in these rocks, and often the rock is high in organic content and the source of the hydrocarbon. Yet because of marginal rock matrix quality, these reservoirs generally require both natural and induced fracture networks to enable economic recovery of the hydrocarbon. Rock types in this class include shale and coalbed methane (CBM.) The term shale is a catchall for any rock consisting of extremely small framework particles with minute pores charged with hydrocarbon and includes carbonate and quartz-rich rocks. Another type of unconventional reservoir is stacked pay units exhibiting somewhat better pore characteristics than in the case outlined above but with the individual units tending to be lenticular in shape and having an extremely small size or volume. These two classes of unconventional reservoirs are amenable to well stimulation and will be the focus of this paper. The above rock types when commercially exploited are known as resource plays. Once a low-priority, the depletion of conventional reservoirs and improving price for oil and gas has driven unconventional reservoirs to an important place in the oil and gas industry. In some regions (i.e., Rocky Mountain province), unconventional reservoirs represent the primary target of current activity and remaining hydrocarbon development. Given their unique petrophysical properties, each type of unconventional reservoir requires a unique approach to well stimulation, with often differing objectives than exist with conventional reservoir types. This paper reviews the characteristics of the basic unconventional reservoir types, lessons learned and successful stimulation practices developed in completing these reservoirs, and areas for improvement in treatment and reservoir characterization and treatment design. Introduction Unconventional reservoirs amenable to hydraulic fracturing are generally hydrocarbon-rich rocks with poor matrix characteristics. By matrix is meant the inter-particle pore network of the rock mass, with pore connections determining the rate of fluid flow from pore to pore or from pore to large flow channel (i.e., solution mold, fracture, or wellbore.) In unconventional reservoirs, pore interconnections are extremely small, significantly reduced in aperture by the liquid wetting-phase, and consequently fluid flow is extremely low. In the case of oil or gas-condensate reservoirs, low mobility of the viscous liquid phase and multi-phase flow worsens the situation. Sometimes, a change in reservoir fluid mobility within the accumulation causes a loss of commerciality and bounds the limits of the pay within the field. This is the case in the in the Codell sandstone (Wattenberg field, DJ Basin, northeast Colorado) as the thermally-influenced in-situ hydrocarbon phase changes from gas to oil along the boundaries of the field. A dense network of natural fractures or a combination of fractures and solution channels with adequate apertures are generally needed to enable flow of hydrocarbons at commercial rates, and drainage of the reservoir to a significant degree. Even with an improved pricing environment, the marginal flow properties and recovery factors of most unconventional reservoirs make necessary a continuous effort to reduce costs and improve efficiencies in all aspects of drilling, completing and producing these wells. Many of the recent improvements and innovations in well completions and hydraulic fracturing have been focused as much on the cost aspect as with improving well productivity.

Demographic response of Louisiana Waterthrush, a stream obligate songbird of conservation concern, to shale gas development

“…The next several sub-stagesin the stage increase the volume of fine-grained proppant while the volume of fluids pumped are decreased incrementally from 50,000 gallons (gals) to 30,000 gals. This fine-grained proppant is used because the finer particle size is capable of being carried deeper into the developed fractures 280 . In this example, the fine proppant sub-stages are followed by eight sub-stages of a coarser proppant with volumes from 20,000 gals to 10,000 gals.…”

Experimental identification of the transition from elasticity to inelasticity from ultrasonic attenuation analyses

“…The presence of fractures, either natural or induced, has a strong control on fluid flow in the earth's crust and becomes increasingly important for oil and gas exploration, geothermal energy, and other geologic near-surface applications (e.g., Curtis, 2002;Cramer, 2008). The production of earth resources is shifting from increasingly scarce conventional resources (traditional highporosity, high-permeability rocks) toward abundant, but more difficult to produce, unconventional reservoirs that contain natural or man-made fracture networks (e.g., fractured carbonates, shale or tight sandstone reservoirs, and deep geothermal reservoirs).…”

Integration of Water Resource Models with Fayetteville Shale Decision Support and Information System

“…As such, developing these resources requires a means to increase permeability through stimulation, specifically hydraulic fracturing (Cramer, 2008). Hydraulic fracturing is a well stimulation technique that has been employed in the oil and gas industry since the late 1940s and is intended to increase the exposed flow area of the productive formation, connecting the fractured area to the well by creating a highly conductive path extending a carefully planned distance outward from the well bore into the targeted hydrocarbon-bearing formation, so that hydrocarbons can flow easily to the well (Schlumberger, 2008).…”

“…The United States production of natural gas is expected to exceed consumption early in the next decade. The United States is projected to become a net exporter of liquefied natural gas (LNG) in 2016, a net pipeline exporter in 2025, and an overall net exporter of natural gas in 2021(AEO, 2012).Shale gas plays are unconventional reservoirs containing gas-bearing rocks which have poor or limited natural permeability relative to the transmission of fluids (Cramer, 2008). As such, developing these resources requires a means to increase permeability through stimulation, specifically hydraulic fracturing (Cramer, 2008).…”