Inversion-based method for estimating total organic carbon and porosity and for diagnosing mineral constituents from multiple well logs in shale-gas formations

Abstract

Reliable estimates of petrophysical and compositional properties of organic shale are critical for detecting perforation zones or candidates for hydro-fracturing jobs. Current methods for in situ formation evaluation of organic shale largely rely on qualitative responses and empirical formulas. Even core measurements can be inconsistent and inaccurate when evaluating clay minerals and other grain constituents. We implement a recently introduced inversion-based method for organic-shale evaluation from conventional well logs. The objective is to estimate total porosity, total organic carbon (TOC), and volumetric/weight concentrations of mineral/fluid constituents. After detecting bed boundaries, the first step of the method is to perform separate inversion of individual well logs to estimate bed physical properties such as density, neutron migration length, electrical conductivity, photoelectric factor (PEF), and thorium, uranium , and potassium volumetric/weight concentrations. Next, a multilayer petrophysical model specific to organic shale is constructed with an initial guess obtained from conventional well-log interpretation or X-ray diffraction data; bed physical properties are calculated with the initial layer-by-layer values. Final estimates of organic shale petrophysical and compositional properties are obtained by progressively minimizing the difference between calculated and measured bed properties. A unique advantage of this method is the correction of shoulder-bed effects on well logs, which are prevalent in shale-gas plays. Another advantage is the explicit calculation of accurate well-log responses for specific petrophysical, mineral, fluid, and kerogen properties based on chemical formulas and volumetric concentrations of minerals/kerogen and fluid constituents. Examples are described of the successful application of the new organic-shale evaluation method in the Haynesville shale-gas formation. This formation includes complex solid compositions and thin beds where rapid depth variations of mineral/fluid constituents are commonplace. Comparison of estimates for total porosity, total water saturation, and TOC obtained with (a) commercial software for multimineral analysis, (b) our organic-shale evaluation method, and (c) core/X-ray diffraction measurements indicates a significant improvement in estimates of total porosity and water saturation yielded by our interpretation method. The estimated TOC is also in agreement with core laboratory measurements.

Geophysical Data: Estimating Organic Matter in Rocks

“…Авторами [4,5] у свій час було запропоновано більш досконалий спосіб визначення вмісту органічного вуг-лецю, пористості та основних породоутворюючих міне-ралів за даними геофізичних досліджень свердловин у газосланцевих формаціях. Суть його полягає у розв'я-занні системи петрофізичних рівнянь, невідомими в якій є вміст керогену (або вміст органічного вуглецю), пори-стість, глинистість і основні породоутворюючі мінерали -кварц, кальцит.…”

Rock Classification from Conventional Well Logs in Hydrocarbon-Bearing Shale

“…Heidari et al (2011) produced and successfully applied a twostep inversion technique that is utilized in this thesis. Layer properties estimated from the joint inversion of well logs are the outputs of the first inversion step and are used for rock typing.…”

“…The methods presented in this thesis are closely tied to core, but attempt to extend core observations to the reservoir as a whole using well logs. The nonlinear multi-mineral inversion technique developed by Heidari et al (2011) is assumed to be a valid method for the estimation of mineralogy from conventional well logs.…”

“…They identify organic content, measured by TOC or kerogen concentration, as measures of gas-shale quality. Heidari et al (2010Heidari et al ( , 2011) developed a deterministic nonlinear inversion method to estimate volumetric concentrations of rock components for applications in conventional reservoirs and shale gas reservoirs. This method was shown to produce reliable quantitative estimates of mineralogy, porosity, kerogen concentration, and saturation when compared to core data.…”

“…This method was shown to produce reliable quantitative estimates of mineralogy, porosity, kerogen concentration, and saturation when compared to core data. Rock-composition quantities presented in this thesis are calculated with the code developed for shale gas applications by Heidari et al (2011).…”