A fluid-fluid interface that joins a solid surface forms a common line. If the common line moves along the solid, a mutual displacement process is involved and is studied here. Some simple experiments motivate the formulation of the basic assumption of the analysis. The basic assumption is a formalization of the idea that the fluid-fluid interface rolls on or unrolls off the solid. This forms an axiom for the mostly kinematical analysis that follows. The predictions are tested through a series of qualitative experiments. The role of the no-slip boundary condition at the solid surface is discussed.

The dynamic contact angle formed when silicone oil displaces air from the surface of glass has been measured. Even though the glass was neither treated in any special way nor cleaned by an elaborate technique, the standard deviation associated with our measurements was approximately 1·5°. A sequence of experiments revealed that the dynamic contact angle depends on both the speed at which the oil spreads across the glass and the size of the characteristic length scale associated with the device within which it is measured. It is shown that the latter implies directly that: (i) the measured and the actual contact angles are not the same; (ii) the usual hydrodynamic model for fluids is inadequate when a moving contact line is present. These conclusions are consistent with recent theoretical studies.

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Permeability is one of the most important parameters for evaluating the production potential of a reservoir. Though there are several ways to determine permeability, the focus here is on formation tester pretests. The pretest is a routine measurement that provides in situ values of formation mobility (permeability/viscosity) at several depths along a wellbore. Because this measurement is easily accessible, it is of interest to maximize the value of the information obtained. Two types of mobility can be determined from a pretest, one of them is the mobility obtained using the amplitude of the pressure signal, usually referred to as drawdown mobility. This measurement has a very shallow depth of investigation; therefore, it characterizes only the formation in the immediate vicinity of the probe, i.e., it is an indicator of the mobility at the wellbore wall, where formation damage is likely to occur. The uncertainty associated with this mobility measurement decreases when the drawdown is long enough to approximate steady state flow. Another way of inferring mobility is done by analyzing the pressure transient signal during the late stage of buildup, during the so-called spherical flow period, hence it is referred to as the spherical mobility. This method has the advantage of characterizing properties deeper in the formation. However, the spherical flow regime is usually difficult to identify in the recorded pressure signal, and in the few instances where it has been claimed to be identified, the inferred formation mobility is almost always smaller than the drawdown mobility. A major problem in analyzing pressure data during the spherical flow period is that it requires quantifying a small rate of change in the signal as equilibrium is approached. In this study, we model the performance of three different tool configurations with six different formations having mobility ranging from 0.001md/cp to 100md/cp, to identify the pretest parameters that maximize the pressure signal during the spherical flow regime. The results may be used as a general guideline on the selection of pretest parameters when the objective is to evaluate formation properties away from the wellbore. When representative values of drawdown and spherical mobility are obtained, it is possible to infer the undamaged formation mobility as well as the magnitude of the formation skin.

To date the analysis of pressure transients associated with formation tester pretests assumes that the temperature remains constant during the measurement. Most fluids experience a decrease in temperature when undergoing an expansion, as in the case of the drawdown; therefore, it is to be expected that the temperature of the fluid within the flowline of the formation tester will change for typical pretest conditions. The object of this study is to assess the impact of these temperature transients on the pressure signal measured with single probe formation testers. The governing equations to analyze pressure and temperature changes in the flowline have been identified. The semianalytic solution presented here couples conductive heat transfer with conservation of mass in the flowline. Sensitivity studies and practical applications are provided to illustrate the importance of thermal effects on the pressure signal recorded during the pretest. It is demonstrated theoretically that in addition to the pretest parameters (rate and volume) and tool parameters (probe size and flowline volume), the specifics of the formation tester design and environmental conditions (pressure, temperature, type of drilling fluid) are relevant to quantify the impact of temperature effects on the measurement of sandface pore pressure with a formation tester. It has been found that the rate of thermal equilibration during the buildup could affect the behavior of the pressure signal. In particular, thermal effects may extend the duration of the flowline storage period. The two major consequences are: 1) longer time could be required for the flowline pressure to equilibrate to the sandface pore pressure; and, 2) it is possible for flowline storage effects to mask the deep formation response, e.g., spherical flow, in the late part of the buildup. The latter could introduce errors in the interpretation of the buildup pressure signal to obtain formation mobility. Other situations of practical interest, such as "false buildups" after a dry pretest, and "builddowns", i.e., a decrease in the pressure signal after an overshoot beyond the sandface pressure during the buildup, are also addressed here.