The paper discusses several issues related to the interpretation of transient pressure tests on horizontal wells. Horizontal well type curves incorporating the effects of wellbore storage and skin are used to aid in the diagnosis of and estimation of parameters from test data. Issues addressed in this study include the identification of flow regimes, validity of Horner graphs. and analysis of multirate tests. An example interpretation is discussed to illustrate concepts.
It is shown that for other than the most ideal situation, the early radial flow behavior will not be evident. In tests with short flow or buildup periods, it is possible for wellbore storage and skin effects to mask both the early radial and linear flow periods. Where early radial flow is present, it is shown that periods. Where early radial flow is present, it is shown that as long as flow length is very long compared to shutin time, a Horner plot may be used to compute formation parameters. In the general case. type curve matching procedures provide adequate means for estimating the desired parameters from the well test.
Several authors have presented equations describing the transient pressure behavior of horizontal well. For a well bounded by upper and lower impermeable boundaries. the following flow regimes have been inferred: early radial. linear. and pseudo radial Fig. 1 is a schematic of the sequence of development of the various flow regimes.
During the early radial period, pressure behavior is infinite-acting, but not necessarily radial. Actual radial flow will occur when the permeability ratio, kv/kH, equals one. Data acquired during this period may be analyzed to obtain a value of equivalent permeability. keq, given by
k eq = kHkV (1)
Horizontal permeability, kH, may be computed from pressure data obtained during linear or late radial flow. To obtain reliable values of both kH and kV, it is required that data be available from the early radial flow period and either the linear or the late radial period.
Depending on well condition and reservoir thickness. the early radial period may be completely masked by wellbore storage and skin effects. The presence of a gas cap or aquifer may distort the linear flow regime, and except in a case where well length is small (relative to reservoir lateral extent), or where flow time is appreciably long, the late radial period may not be observed. For obvious practical reasons, afterflow in horizontal wells will always be present, corresponding at a minimum, to the lateral section of the well. In the general case type curve matching procedures offer the best means of identifying flow regimes, and obtaining reservoir parameters.
It is important to note that from a physical and mathematical viewpoint, the pressure behavior of a horizontal well is equivalent to that of a partially completed vertical well bounded by parallel faults. Where pseudoradial flow effects have not been parallel faults. Where pseudoradial flow effects have not been felt, horizontal well behavior can be shown to be identical to a fully penetrating vertical well bounded by parallel faults. In this rotated point of view the parallel faults represent the upper and lower boundaries of the original reservoir system.