Characterizing Multi-Layered Reservoirs Using A New, Simple, Inexpensive and Environmentally Sensitive Pre-Production Well Test
Authors: Aly Ahmed , Lee W. John 
Published: December 2007

Abstract
This paper presents a simple, inexpensive, and environmentally sensitive method to characterize and model multi-layer reservoirs with unequal initial pressures in individual layers. The testing techniques involve monitoring pressure data caused by crossflow between the layers prior to flow at the surface (pre-production well testing). When boundary effects are reached in the test, it is possible to estimate individual layer permeability, skin and drainage area. Also, as there is no production on the surface, the new well test is well suited for offshore wells or for testing wells in environmentally sensitive areas.

Introduction
All natural formations are vertically heterogeneous to some degree because of the stratification accompanying their depositional origin. The vertical sequence of deposits is frequently such that layers having good permeability alternate with layers having poor permeability. A sequence of sands interbedded with clay or shale is an example of such a formation. In many cases, we perforate wells simultaneously in two or more layers. The major drawback of the multiple-layer completion concept is that the origin of the produced fluid is generally unknown. As a result, it is difficult to apply reservoir engineering principles to predict recovery and future performance under primary and enhanced recovery operations from each of the layers. Normally, we must determine the properties of the individual layers to improve the prediction of reservoir behavior.
Several studies have examined the wellbore response in multilayer reservoirs. However, most of these studies examined wellbore response from a two-layer reservoir with equal initial pressures. In this work, we expand the scope of study to investigate the wellbore response from multi-layered reservoirs (two-, three- and five-layers) with unequal initial pressures. We address the needs of the industry, especially in North Sea reservoirs, for practical and easily applied methods to determine individual layer properties by modeling and analyzing the performance of multi-layer reservoirs with unequal initial pressures.
The first phase in layered reservoir studies dealt with the problem of commingled reservoirs with equal initial pressures. The second phase included the effect of unequal initial layer pressures which Papadupolos studied first for layered aquifers. Larsen presented a method for analyzing wellbore pressures prior to the start of production for a two-layer reservoir, provided such data are available from the infinite-acting period. The method has the disadvantage that it requires estimates of the average permeability, porosity and compressibility for each individual layer. Also, this method can yield only an estimate of the average reservoir properties and not individual layer properties. Kuchuk et al. presented generalized analytical solutions for commingled reservoirs in which each reservoir or layer can be at a different initial pressure or may have a different initial pressure distribution. Agarwal et al. presented a detailed study of the preproduction time period. The study showed that much information that has bearing on production-performance can be discerned by the observation of the pressure behavior during the pre-production time period. A number of approximate solutions were presented for analyzing well responses. Aly presented a complete study of the performance of commingled reservoirs with unequal initial pressures. Aly et al. presented a new method for analyzing the PPWT pressure data from a two-layer, commingled reservoir with unequal initial layer pressures to determine the individual layer properties. Aly et al. presented a detailed development of the mathematical model for multilayer reservoirs with unequal initial pressures.

Pre-Production Well Test Design
The PPWT is performed early in the life of a reservoir, when the information is most needed for planning production schedules and making economic decisions. Pre-production is the period after completion but before production of the well. Immediately after perforation, we position a pressure gauge above the top perforation to measure the pressure performance of the total system. The differential pressure between the layers causes cross flow from one layer to another which triggers the pressure signal. One important advantage of the preproduction well test is that there is no surface production during the test. Thus, the environmental impact caused by flaring oil or gas is eliminated.
The pre-production well test needs only a pressure gauge above the top layer and the pre-production pressure data up to the late-transient region. The cost and the time are reduced substantially compared to the commercial "Layered Reservoir Test."
The PPWT is well suited for offshore wells and for environmentally sensitive areas like the Gulf of Suez, as there is no production on surface during the test. Also, because this test is carried out very early in the life of the reservoir, before the pipelines and facilities are ready, no production time is lost. Fig. 1 shows a sketch of the pre-production well test. A schedule for the test follows:Record the initial pressure of each layer using a wireline formation tester.
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