Due to economics or time constraints, well-testing is sometimes reduced to perforating a well under-balanced, andanalyzing the inflow characteristics. The objective of a Perforation Inflow Test Analysis (PITA) is to estimate the initial reservoir pressure, permeability and skin, immediately after perforating the well. This information can be used for evaluating future development strategy. However, special analytical procedures are required for analyzing the data, because these perforation inflow tests are shorter than conventional well tests and the influx rates are not measured.
In this study, the working equations for analyzing these short tests are presented, and the procedure required for calculating meaningful estimates of the reservoir parameters is presented. Analyses of early-time and late-time data are the two major components of this approach. The early-time analysis isused for estimating the skin, and the late-time analysis is used for estimating the initial pressure and permeability. A distinctive feature of the PITA is that it does not require calculation of the influx rates, which are generally not available during a perforation test. A special derivative, called the impulse derivative, can be used to determine if the data collected is sufficient to yield meaningful results from a PITA. It is particularly important that the reservoir-dominated flow regime be reached, if the estimates of initial reservoir pressure, permeability and skin are to be acceptable. Good estimates of these parameters from a PITA will minimize the uncertainty associated with non-uniqueness in inverse problems, when modeling the test data.
Conventional well tests have served the petroleum industry faithfully for decades as the primary and most reliable means of:
quantifying deliverability,
characterizing the reservoir,
collecting reservoir fluid samples, and
evaluating the condition of the well.
However, for the last few years, oil and gas producers have been searching for alternatives that could yield the desired information in less time, in a more environmentally-friendly manner, and at a cheaper cost than from conventional well tests. The trend has inevitably been towards tests of shorter duration. Although it is accepted that results from short tests with small radii of investigation may not be as reliable as those from conventional well tests, it is reasonable to accept that they could be of value in assisting with strategic decisions about field development, when an increased margin of error can be tolerated.
In offshore wells, in addition to the potentially exorbitant cost of testing (several millions of dollars), the drive towards green (shorter) tests is fuelled by environmental considerations, such as requirements for restricted flaring of hydrocarbons. InAlberta and elsewhere in North America, the driving force towards inexpensive tests is the marginal economics of low deliverability wells. Either way, there is an increasing trend towards these green tests to replace conventional well tests. One such green test consists of simply allowing the well to flow into the closed wellbore after perforating (closed chamber test). As the fluid from the reservoir enters the wellbore (with a fixed volume), the wellbore pressure builds up.
