This paper summarizes the analysis of wireline formation tester (WFT) data collected in tight gas sands.
An interpretation algorithm based on type-curve matching is used in me analysis. The type curves are constructed using an analytical model. The model assumes hemispherical flow geometry and incorporates the decompression period and flowline storage effect.
The permeability values calculated using this algorithm agree reasonably well with the permeability values available from core and well log analysis.
Pressure data from WFT have been used to determine initial reservoir pressure, vertical pressure distribution, fluid contacts, and formation permeability. Formation permeability is deduced from the pressure behavior observed during either the pretest phase1–4 or the sampling period.9,10,13 Whether the flow is into the pretest chamber or into a sampling tank, the fluid flow into a WFT tool is three dimensional and has a convergent flow pattern. The inrerpretation models currently used to calculate formation permeability usually assume spherical or radial flow geometry. The shortcomings of the assumed simplified flow configurations have been documented in the literature. 15 In recent years, several three dimensional (3-D) WFT models have been presented. 11–17 These models take into account the 3-D flow geometry but most of them consider constant flow rate at the sandface.
In addition to the flow geometry, several other factors complicate the modeling and analysis of WFT tests:
decompression prior to formation flow;
flowline storage effect; 16
supercharging effect 16
filtration around the tool packer, and
air trapped in flowlines.
This paper outlines a hemispherical analytical model. The model incorporates the decompression period and flowline storage effect to account for flow rate change at the sandface. Type curves generated from this model are used to analyze WFT data gathered from tight gas sands.
The pressure in the WFT tool prior to the test is the hydrostatic mud pressure. The drawdown starts from me hydrostatic mud pressure and me formation does not flow until the pressure in the tool is lowered below sandface pressure. The period from the beginning of the test to the Start of formation flow is referred to as the decompression period. During this period, only the decompression of the fluids due to volumetric enlargement of a closed chamber takes place.
Air and drilling mud may exist in the flowline. The existence of trapped air as a separate phase complicates the analysis of WFT data substantially.
The simultaneous decompression of air and drilling fluid can be formulated using the mass conservation principle.
Equation (available in full paper)
The sensitivity of the pressure response to the volume of trapped air 1s investigated using Eq.1. The results of the investigation, in which air is considered as an ideal gas and the data of test 8 in Glasgow #2 is used is displayed in Fig.I. As expected, when there is no gas in the chamber, a perfect straight line is obtained.