Thinly bedded reservoirs are increasingly a target of offshore exploration in the Malay Basin. These reservoirs exhibit heterolithic interbedding with vertical heterogeneity and a wide range of layer flow properties. This paper describes methods of real-time and high-resolution formation evaluation and formation testing used to characterize such reservoirs.
The formation evaluation of thinly bedded reservoirs has several objectives:
identify the layers that may contain hydrocarbons
verify productivity and fluid types with formation testing and sampling
calculate net pay thickness and uncertainty range.
The evaluation is complex because of bed geometry and lithology. The reservoir beds are often thinner than the resolution of the formation evaluation logs. They exhibit a silty lithology and fine grain texture, and require high quality borehole resistivity images to characterize their geometry. The exploration well offers the best chance to evaluate the prospect, but operational and economical constraints do not permit to core or test every potential reservoir. Therefore, a real-time evaluation must identify with certainty the prospects to test.
The sequence of real-time formation evaluation described here consists of the following steps.
Wellsite petrophysical analysis of:
volumes of clay, minerals and fluids
permeability based on the combination of GR spectroscopy elemental yields, nuclear and sonic porosity, and array resistivity logs.
Identification of thin beds, and computation of sand-count from an electrical borehole resistivity imager.
Validation of the wellsite petrophysical analysis and the borehole image within two hours of recording the logs, and design of the wireline testing and sampling program.
A modular wireline formation tester is used to measure accurate in-situ pressures in potential reservoir rock and to establish fluid gradients and identify contacts. Additionally, local formation permeability is estimated from analyses of pretests, and is used to verify the productivity of thin beds identified by formation evaluation techniques.