Pressure build-ups from properly conducted drill stem tests have provided the well owner an ever increasing amount of useful data in regard to formation evaluation. The distance to a discontinuity is a significant formation parameter that can be calculated from drill stem test data. This study investigates the various methods available for calculating the approximate distance to a discontinuity. The transient response curves obtained during a drill stem test exhibit more than one slope if the flow periods and shut-in periods are sufficiently large. The distance to the discontinuity can be calculated by picking the point of deviation from the straight line portionT +0 of a well bore pressure vs. log ------ plot.0
Calculations are shown for a theoretical example of a build-up curve and two field examples of drill stem tests.
A need has existed for an accurate method of determining the distance from a well bore to a discontinuity. Many advantages will be afforded the well owner if he can determine the distance to a discontinuity. For, example, a much higher permeability section may be indicated at some distance from permeability section may be indicated at some distance from the well bore as shown in Figure 1. Calculating the distance to the higher permeability section will provide the answer as to whether or not the section can be reached with a stimulation treatment such as hydraulic fracturing. Also, assume a test of a water bearing section indicates a gas-water contact as shown in Figure 2. The distance to the contact will determine whether a new well must be drilled higher on the structure or whether a side track from the original hole is feasible for penetrating the formation above the gas-water contact. Additional examples of common discontinuities are shown in Figures 3 and 4.
One of the prime objectives of a drill stem test is to determine the flow capacity of the zone being tested. The flow capacity is directly related to the transmissibility of the formation which is directly related to the effective reservoir permeability and effective formation thickness, and inversely permeability and effective formation thickness, and inversely proportional to the viscosity of the flowing fluids. The flow proportional to the viscosity of the flowing fluids. The flow capacity is further affected by the amount of well bore damage present. present. Drill stem tests are usually comprised of a sequence of flow periods followed by shut-in periods. The shut-in periods (build-up curves) provide the basic data which is utilized in calculating values for transmissibility and well bore damage.
Transmissibility is obtained from a plot of the build-upT + 0 pressure versus the log ----- as shown in Figure 5. pressure versus the log ----- as shown in Figure 5. 0 Theoretically, the points will fall on a straight line with a slope of m psi/cycle. Extrapolation of that straight line to T + 0 ------ = 1 gives the static reservoir pressure. The 0 transmissibility is obtained from the relationship:
Kh 162.6qB----- = ---------m
Where K = effective permeability in md.
h = effective formation thickness in ft.
= flowing fluid viscosity in cp.
M = slope of the build-up curve in psi/cycle.
B = formation volume factor.
q = flow rate prior to shut-in, bbls/day.