Interpretation of Wireline Formation Tester (WFT) Data In Tight Gas Sands
Authors: Yildiz T. , Bassiouni Z. 
Published: March 2010

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 the analysis. The type curves are constructed using an analytical model. The model assumes hemispherical flow geometry and incorporates early time decompression period and flowline storage effect. The permeability values calculated using this algorithm agree reasonably well with the permeability values available from core measurements.

Introduction

Pressure data from WFT have been used to determine initial reservoir pressure, vertical pressure distribution, fluid contacts, and formation permeability. A WFT tool typically has a probe, piston, pressure gauges, one or two pretest chambers, sampling tank, and flowlines. Figure 1 gives a schematic of the WFT tool with one pretest chamber.

Formation fluid is withdrawn into either pretest chamber or sampling tank. Before the test starts, the tool is subjected to the drilling fluid hydrostatic pressure. The test is started after the packer on the tool isolates the formation from the drilling mud in the wellbore. The probe is inserted into the formation through the mudcake. When the tool is used in pretest mode, the piston is pulled at a specified constant rate, and formation fluid is withdrawn into the pretest chamber through the flowlines. During this drawdown period, the pressure in the tool decreases continuously. Once the piston movement is stopped, formation fluid continues to flow into the tool. The pressure in the tool rises gradually until it reaches formation pressure.

A wireline formation test consists of three periods: decompression, drawdown, and buildup. During all three periods, the pressure in the tool is recorded. Figure 2 illustrates the piston displacement rate, sandface-flow-rate and the expected pressure response for an ideal WFT test conducted in a low-permeability environment. The pressure in the WFT tool prior to the test is the hydrostatic mud pressure (Pmi), which is higher than formation pressure (Pi) After the piston is activated, the chamber volume increases and the fluid in the tool decompresses. Flow from the formation may start only after the pressure in the tool is lowered below the formation 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, the pressure decline is solely due to the decompression of the fluid in the flowlines. The drawdown period starts when the tool pressure reaches the formation pressure. In the drawdown period, the pressure response is a function of the fluid decompression and formation flow. Because of limited flow from the low permeability formation, the pressure response is dominated by fluid decompression. Once the piston is stopped, the buildup period starts and the formation-fluid-flow into the tool compresses the formation and flowline fluid back to pretest conditions. The pressure in the tool gradually builds up to initial formation pressure, provided the test lasts long enough.

Formation permeability is deduced from the pressure behaviour observed during either the pretest phase(1–4) or the sampling period(5–7).