Methodology for Petrophysical and Geomechanical Analysis of Shale Plays. Case Study: La Luna and Capacho Formations, Maracaibo Basin.

摘要

According to EIA (Kuuskraa et al., 2013) and PDVSA Exploration (Molina et al., 2014), the Maracaibo Basin in Western Venezuela has two stratigraphic units of Late Cretaceous age which are considered as important unconventional shale plays in Latin America: La Luna Formation and La Grita Member of Capacho Formation. Venezuela is ranked in 7t place in shale oil resources, with an estimate above 13,000 MM barrels, and ranked in 13th place in shale gas resources, with an estimate above 201 TCF. In order to evaluate the mentioned shale plays, a methodology was developed through the integration of petrophysical, geochemical and geomechanical analysis. The complete evaluation methodology has 4 phases: 1) petrophysical evaluation, that includes multimineral evaluation, porosity estimation and calibration with mineralogical analysis; 2) TOC content evaluation, that includes TOC content estimation, using 3 methods: density logs (Schmoker, 1979), gamma ray logs (Schmoker, 1981), sonic and resistivity logs (Passey et al., 1990), and porosity associated with organic matter; 3) geomechanical evaluation, that includes rock mechanical properties estimation, state of stress determination and brittleness index estimation, using 3 methods: mineral content (Jarvie et al., 2007), elastic properties (Rickman et al., 2008) and a modified method of Grieser-Bray (2007), calibrated with geomechanical field test (XLOT, microfracs and minifracs); and 4) determination of most prospective intervals for completion. La Luna Formation and La Grita Member of Capacho Formation are mainly composed by carbonatic rocks, with high content of calcite (above 75%) and low content of clay minerals. In both units, the estimation of TOC content varies from 0.50 to 9%. Mechanical properties show moderate values of Poisson's ratio (0.20 to 0.32), high values of Young's modulus (0.80 to 9.60x 10~6 psi) and UCS (6.20 to 31.00x 10~6 psi). In the Cretaceous sequence, the state of stress changes according to geographic location in the basin, from normal in northwest region and central lake region, to transcurrent and reverse in southeast region. The brittleness index estimated for different methods varies from 0.54 to 0.85, which indicate that both units may be classify as brittle. The integration of geomechanical and petrophysical analysis allowed identifying prospective intervals in both units, with thickness between 20 to 100 ft. Therefore, the study indicates that both units show very good conditions for horizontal drilling and hydraulic fracturing. Moreover, the comparison of various estimation methods of TOC content and brittleness index allowed to observe the uncertainty presented by these parameters in analysis of shale plays.