AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Rock-Physics-Based Permeability Heterogeneity and Fluid Evaluation of an Ultra-Deep Low-Porosity Carbonate Reservoir, Sichuan Basin, China: I. Petrophysical Study
(1) Texas A&M University, College Station, TX.
(2) Sinopec Southern Exploration Company, Chengdu, China.
(3) Sinopec, Beijing, China.
Owing to the high-pressure and high-temperature environment and the complex history of diagenesis, ultra-deep reservoirs with depth of greater than 5 km usually exhibit much higher degree of reservoir heterogeneity than their counterparts in shallower depths. In the recent discovered gas fields, Sichuan Basin, China, reservoir units could have variable porosity ranging from less than 5% to greater than 20%, while producing units having a porosity of less than 5% are most common. Prediction of the occurrence of high-quality reservoir zones has been one of the many greatest challenges encountered in ultra-deep exploration.
Combing geological description and core analysis with rock-physics-based petrophysical study, we find that in low-porosity reservoir rocks (porosity <5%), variations of pore type play an important role in causing complexity of permeability-porosity relationship and spatial variations of gas occurrence. A frame flexibility factor defined in a new rock physics model is found to be useful in classifying different pore types and quantifying their effects on acoustic wave velocity and permeability heterogeneity in low-porosity carbonate reservoir rocks. For the studied reservoirs, it is found that samples with frame flexibility factor <2.2 represent a mixed pore type of intercrystalline and interparticle pores where porosity is < 5%, or a mixture of interparticle and dissolution pores where porosity is > 5%. Samples with frame flexibility factor between 2.2 and 6 show either intercrytalline pore space in low porosity range (<5%) or dominant interparticle pore space in relatively high porosity range (>5%). Fractures usually have high frame flexibility factor of > 6. It is observed that low-porosity (<5%) gas-bearing reservoir rocks with intense dolomitization tend to have lower frame flexibility factor and higher sonic velocity than water-saturated rocks in the similar porosity range.
It is also found that frame flexibility factor could be a good indicator to differentiate relatively high permeability zones (>~0.1md) from non-reservoir zones (average permeability < ~0.001md). Once this frame flexibility factor is calibrated using core and petrophysical data, it can then be estimated from log and seismic data to help determine high-permeability gas-bearing zones in low porosity carbonate reservoir rocks.