AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Challenges in Formation Evaluation of Tight Gas Reservoir
(1) G&G Petrophysics, Reliance Industries Ltd (E & P), Navi Mumbai, India.
Unconventional reservoirs like shaly sand, laminated sand shale and tight reservoirs always pose challenge to the log analyst, as the conventional methods fail to address the problems specific to these reservoirs. Tight gas reservoirs often have low porosity; therefore, saturation estimated will mostly be low. The situation may lead to grossly underestimating the reservoir potential. Situation becomes more complicated when heavy minerals like pyrite and siderite are present. The resistivity recorded will be affected due to the presence of conductive minerals. Conventionally, low resistivity reservoirs reads higher resistivity but presence of conductive minerals lowers the resistivity. Sometimes resistivity is low in the middle of the reservoir with maximum porosity in that zone. This leads to a peculiar situation showing very high water saturation in those intervals, misleading to the geoscientists. Density neutron logs, unless calibrated with the mineral volumes, will under estimate the true porosity of the reservoir. Therefore, conventional petrophysical interpretations will always underestimate the reservoir potential.
Uncertainty in hydrocarbon pore volume estimation in tight gas reservoir can be minimized by using NMR and Spectroscopy data in conjunction with conventional logs. The present study was carried out in well drilled in deep water, east coast of India with SOBM. This well was logged with all conventional tools and some advance tools like nuclear magnetic resonance (NMR), elemental capture spectroscopy, XPT and dual packer formation tester. This paper present different petrophysical approaches for formation evaluation in tight gas reservoirs and quantify the uncertainty associated with each method. The stochastic inversion modeling (by incorporating heavy minerals), NMR technique, integration of elemental capture spectroscopy data with conventional logs and integration of NMR data with conventional logs were carried out to address the problem. The study proved very useful in unlocking the reservoir potential and helped in reducing the uncertainty upto a great extent.