Analysis of Pressure Build-Up Data
- G.B. Thomas (Signal Oil and Gas Co.)
- Document ID
- Society of Petroleum Engineers
- Journal of Petroleum Technology
- Publication Date
- April 1953
- Document Type
- Journal Paper
- 125 - 128
- 1953. Original copyright American Institute of Mining, Metallurgical, and Petroleum Engineers, Inc. Copyright has expired.
- 2 Well Completion, 4.1.2 Separation and Treating, 4.3.4 Scale, 2.4.3 Sand/Solids Control, 5.2.1 Phase Behavior and PVT Measurements, 5.2 Reservoir Fluid Dynamics, 4.1.5 Processing Equipment, 1.8 Formation Damage
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Several methods of analyzing pressure build-up data in wells have beenpresented by various authors. This paper reviews the theory and method of D. R.Horner and presents example calculations performed on data obtained by testingseveral different types of wells. These calculations include, (1) graphicalestimation of final static pressure, (2) determination of the productivecapacity of the pay away from the well bore and, (3) the degree to which theformation adjacent to the well bore has been damaged by completion or othercauses. The methods of testing and precautions which should be taken to assurethe best data possible are discussed. Limitations and reliability of calculatedresults are also treated.
Pressure testing of wells is generally limited to the determination ofproducing and static mean formation pressures. The so-called "static"pressure determinations, along with PVT, electric log and production data,enable the reservoir engineer to determine, within reasonable limits, the drivemechanism of the reservoir and in some cases, the amount of edge waterencroachment. Producing pressure tests enable calculation of productivityindices and allow the engineer to plan the systematic production of a poolfor
optimum conservation of subsurface energy.
The radial flow formula advanced by Muskat has been based on the assumptionof incompressible radial fluid flow. It has been known that reservoir fluids donot behave in an ideally incompressible manner. For example, incompressibleflow theory indicates a simple logarithmic relationship between the differenceof the instantaneous and static well pressures when plotted against time. Thelatter stages of this type of plot of pressure build-up data generally show amarked deviation from the earlier straight line trend, which deviation may beshown to be due to the compressible flow of fluids toward the well bore.Shut-in times of 24, 48, 72, or at most 96 hours are currently in wide use fordetermining so-called "static" reservoir pressure. Due to thecontinuation of compressible flow of fluids into the well bore long after thisarbitrarily taken shut-in time, the determination of static pressures hasalmost invariably resulted in lower than equilibrium values. Materials balancecalculations made early in the life of a reservoir often result in a calculatedreserve which later observations prove to be too low. Failure to obtainreliable "static" reservoir pressures within the prescribed 24 or48-hour build-up period has undoubtedly been a major factor in obtaining theselow estimates. Comparison of theoretical and actual productivity
performance has indicated that formation damage and not being able to attaintrue static pressures have been partially responsible for the observeddiscrepancies.
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