The Marcellus Formation is a shale reservoir in eastern North America whose natural gas reserves are accessible primarily through hydraulic fracturing. Over 100 fracture wells have been stimulated in the shale’s Pennsylvania region, “resulting in an in-depth understanding of details needed to achieve optimal frac performance” (Houston et al., 2009). Type and concentration of fracturing fluid, drill cutting placement, geochemical controls, proppant and perforation strategies have all varied significantly from the beginning of Marcellus Shale extraction to the present. These important factors of hydraulic fracturing have been tested and evaluated in order to capture the best hydraulic fracturing practices for the Marcellus Shale formation. By studying the effects of fracturing strategies and technologies used for these test wells, more rapid advancement towards full-scale natural gas extraction in the Marcellus Shale region can be achieved.— Caitrin O’Brien
Houston, N, Blauch, M, Weaver, D, Miller, D, O’Hara, D. Fracture-Stimulation in the Marcellus Shale- Lessons Learned in Fluid Selection and Execution. Society of Petroleum Engineers, 2009 SPE Eastern Regional Meeting, 23-25 September 2009, 1-11.
Nathan Houston and colleagues from the Society of Petroleum Engineers reviewed methods and technologies used in the Marcellus Shale from early development of the field to the present. By evaluating and capturing the best practices of hydraulic fracturing in this reservoir, the scientists hope to develop standing operating procedures that can be put towards full-scale development of natural gas extraction in the Marcellus Shale. Houston and his colleagues considered practices from hydraulic fracturing in other shale reservoirs, as well as several new techniques that have been developed specifically for the Marcellus. The scientists reviewed the best type of fluids used to stimulate fractures while minimizing runoff and leaks, as well as the best methods of analyzing fracture placement for drill cuttings. Houston and his colleagues also analyzed types of geochemical controls that are used in the fracturing fluid in the Marcellus Shale, such as biocides, scale control and iron control, as well as the results of using different types of surfactant, sand, and perforating and fracture techniques. After reviewing the results of different methodologies, the scientists determined how these each factor interacts with the unique geologic composition of the Marcellus Shale, and made decisions as to which technology or fluid composition was best suited to hydraulic fracturing in this region.
The Marcellus Shale formation is characterized by low permeability rock that has high amounts of organic matter and clay, very fine grain size, and extremely fine porosity. This combination of traits has required hydraulic fracture stimulation fluids with low viscosities, high rates of flow, and large quantities of proppant to hold fractures open. Houston and his colleagues determined that the most effective fracturing fluids for the Marcellus Shale region are slickwater stimulation fluids pumped at high rates, with low sand concentrations. This combination minimizes leakoff, and when combined with a liquid-polymer additive, the slickwater reduces friction in the shale fractures. Analysis of drill cuttings is an important technology to optimize fluid design and fracture placement, and X-ray technology was found to be the best method of identifying the unique mineralogy and fracture geometry of fracture in the reservoir. The scientists found that including several geochemical controls in fracturing fluids is crucial to optimize fracture production and minimize damage to the shale. In particular, “environmentally responsible” biocides that kill sulfate-reducing and slime-forming bacteria are useful to inhibit any potential damage to the shale. Similarly, geochemical precipitants and scale control are useful to protect against carbonate, sulfate and iron-based scale build up in fractures, which can reduce productivity. Houston and his colleagues also determined the most useful concentration of surfactant and proppant, as well as the most effective rate of proppant to be used in the Marcellus Shale. The use of surfactant in the Marcellus Shale fractures has been found to reduce surface tension and lower the pressure in the fractures, resulting in increased recovery of natural gas. White sand has evolved to be the proppant of choice in the Marcellus reservoir, and sand concentrations between 0.25 to 2.5 lb/gallon of fluid have been proven, through trial and error, to be the best concentrations to prop fractures open. Finally, Houston and his colleagues determined the best perforation and fractures for the Marcellus Shale to be those made using deep-penetrating and cleaner-hole technologies, to reduce fracture initiation and breakdown pressures. A mixture of 7.5% hydrochloric acid has proven most effective in reducing friction in the fractures and enhancing performance. By reviewing the lessons learned from the stimulation of more than 100 shale gas wells, Nathan Houston and colleagues from the Society of Petroleum Engineers were able to outline the most effective methodologies for natural gas extraction in the Marcellus Shale region.