An Improved Method To Predict Future IPR Curves
- M.A. Kilns (Chevron U.S.A. Production Co. Inc.) | J.W. Clark III (Chevron Petroleum Technology Co. Inc.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- November 1993
- Document Type
- Journal Paper
- 243 - 248
- 1993. Society of Petroleum Engineers
- 5.1 Reservoir Characterisation, 5.6.8 Well Performance Monitoring, Inflow Performance, 4.6 Natural Gas, 3.1 Artificial Lift Systems, 4.1.5 Processing Equipment, 5.2.1 Phase Behavior and PVT Measurements, 4.1.2 Separation and Treating
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This paper presents a significantly improved yet simple method to predictfuture oilwell deliverability and inflow performance relationship (IPR) curves.For the 21 reservoirs performance relationship (IPR) curves. For the 21reservoirs studied, current empirical techniques overpredicted futureperformance by 117%, while the new approach reduced the average performance by117%, while the new approach reduced the average error to only 9%, This newmethod. when coupled with nodal analysis. could affect equipment sizing,investment planning, and property sales economics significantly because itprovides more property sales economics significantly because it provides morerealistic predictions.
An important engineering toot for maximizing future financial return throughdesign optimization is the ability to develop a family of future IPR curves fora given well or field. These curves may be able to provide answers to suchquestions as tubing and choke size, timing of artificial lift, future revenuestreams. and abandonment time with some certainty. Currently, three simplehand-held methods 1-3 are used to estimate future absolute-open-flow (AOF)rates for wells producing from solution-gas-drive reservoirs. After severalmaximum-rate/ reservoir-pressure pairs have been established, these valuesnormally are coupled with Vogel's dimensionless IPR equation to create afancily of future IPR curves. However, these methods appear to introducesignificant error into the estimation process. First, we will describe thecurrent methods to predict future maximum flow rates. Fetkovich presented arelationship between oil flow rate, average reservoir pressure, and flowingbottomhole pressure (BHP) by pressure (BHP) by (1)
where the flow exponent, n, is assumed to be constant throughout the life ofthe reservoir and the flow constant. J.), varies according to
J -flow constant at current reservoir pressure, pr , and J2 +flow constantat a future reservoir pressure, Pr2. Therefore, with a three- or four-pointflow test, n and J can be estimated for that test and that reservoir pressure.and any future maximum now rate can be projected by
In a second approach, Eickmeier coupled Fetkovich's work with Vogel'sequation and set the flow exponent to a fixed value of 1.0-to arrive at
Instead of a multipoint test like that needed to implement Fetkovich's Eqs.1, 2. and 3, a one-point test can be coupled with Vogel's equation to estimate(q ) . Then, for any selected future reservoir pressure, the correspondingmaximum open-flow potential, (q ) , can be predicted with Eq. 4. potential, (q) , can be predicted with Eq. 4. The third method is Uhri and Blount's"pivot-point" approach, which requires two separate flow-rate tests attwo different average reservoir pressures, Their numerical solution requiresthat two flow constants be determined from the two flow-rate tests suchthat
The maximum flow rate for any given reservoir pressure is then determinedfrom
These three simple methods are available to estimate future maximumopen-flow rates for wells under solution-gas-drive. These AOF values, coupledwith Vogel's equation. can be used to estimate future IPR's. The Eickmeierapproach requires a single-point flow test to implement. the Fetkovich methoduses a multi-point test, and the pivot-point procedure needs two single-pointtests taken at different times.
IPR Data Development
Before describing this paper's new method of estimating future AOF's andhence future IPR performance, discussion of the development of thepressure/flow database used is appropriate. Klins and Majcher give a morecomplete description, Inflow performance of 21 theoretical solution-gas-drivereservoirs was simulated with the Weller method. Table 1 shows that thesereservoirs contain a wide range of rock and fluid properties, relativepermeability characteristics, and skin effects. To construct the greater than19,000 flow-rate/pressure-point data set, Weller describes the pressuregradient as
The oil saturation at any time and location can be estimated from
The fractional recovery, Np/N, is calculated with the Muskat method. Eqs. 8and 9 allow stepwise calculation of pressure and saturation profiles for aspecified now rate from the outer boundary to the wellbore. To conservecomputer time. and because pressure gradients get gradually steeper approachingthe wellbore, pressure gradients get gradually steeper approaching thewellbore, a variable stepping procedure was incorporated. At any point greaterthan 200 ft from the wellbore. a 1.0-ft step was used; between 100 and 200 ft.a 0.5-ft step: between 10 and 100 ft. a 0.05-ft step; and within 10 ft of thewellbore, a 0.01-ft step. The variable stepping procedure was checked bycomparing its results with those obtained with a constant radius step of 0.01ft; results were virtually identical. This procedure produces an accuratesolution while markedly reducing computation time.
Because Weller did not account for skin in his formulation, the method hadto be adjusted to simulate the performance of damaged or improved wells.
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