Rola geofizyki wiertniczej w określeniu zasobów gazu ziemnego w łupkach

Abstract

Application of wire log analysis for petrophysical evaluation and determination of shale gas reserves.A b s t r a c t. The paper presents differences between conventional and shale gas plays (Tab. 1). Shale gas concept comprises a wide range of reservoirs, from the coalbed to tight gas plays. In Europe, where the shale gas rush has just started, shaly rocks were treated so far as unproductive and high quality data sets necessary for evaluating properties of those rocks are usually missing. Therefore, US shale gas plays like Barnett and Haynesville are widely used as valuable reference tools (Jacobi, 2008; Parker, 2009). Coal, where gas is essentially stored entirely by sorption, represents one end of the unconventional gas spectrum and tight gas sands, where gas is essentially stored by compression only — the other end of that spectrum. In turn, shale reservoirs with gas entrapped by sorption and compression, fill the space between the two endpoints. Differentiation of those two components is one of the primary goals of an analysis program. Shale gas reservoirs are formed by a wide variety of rock types which makes it necessary to use most appropriate technologies to characterize both coalbed and tight gas reservoirs. The current paper concentrates on tools for evaluating petrophysical parameters, most suitable for shale gas plays. In the case of old wells with old fashion Soviet logs, the uncompensated neutron gamma tool was commonly used tool. This was the only porosity reading curve in log suite, “neutron porosity curve” which could be overlaid with natural gamma ray (GR) (Fig. 1). Natural gamma ray curve is a good indicator of organic matter, which adsorbed uranium. Other hydrocarbon signatures can be traced on the basis of SP vs GR, GR vs resistivity. Some of hydrocarbon signatures can be related to TOC from core lab measurements. For contemporary good quality wire line log curves the Passey et al. (1990) method has been applied. This method is based on computation of separation between acoustic transit time and resistivity (ΔR) (Fig. 2). The resulting difference is used to calculate TOC taking into consideration maturity of organic matter which is parameter for a bunch of relationships (TOC vs ΔR). Local calibration ΔR to TOC from cores are required. In order to determine reliable relationships between ΔR, gas contents to TOC, the high technology coring service and sensitive laboratory measurements are necessary. The results of petrophysical analyses are important for estimations of gas resources in shales. The formulas for computation of conventional and unconventional gas reserves are generally similar. However, in the case of the unconventional gas reserves, instead of porosity reservoir storage the rock density is applied, and for determinations of hydrocarbon volume—the gas content is applied in place of hydrocarbon saturation. If European unconventional reservoirs turn to be profitable then continent landscape will also change. The big gas fields would require dense networks of rigs that will have some negative environmental impact. This would require a change in industry structure, as well as in public opinion and legal regulations.