Estimation of Reserves Using the Reciprocal Rate Method Conference Paper uri icon

abstract

  • Abstract In this work we develop, validate, and apply the "reciprocal rate method" to estimate oil reserves using only rate-time production data. This approach requires the development of boundary-dominated flow, and can be used to validate reserve extrapolations from numerical/analytical reservoir models. The methodology does presume that flowing well bottomhole pressures are approximately constant — but we will demonstrate that the method is tolerant of substantial changes in the flowing bottomhole pressure. This approach requires a plot of the reciprocal of flowrate (1/q) and the so-called "material balance time" (cumulative production/flowrate or Np/q). The "secret" to this approach is the use of material balance time — this function accounts for most variation in rate/pressure, and permits the extrapolation of the 1/q function. This methodology has been applied for oil wells (including oil wells with high water production) — and in all cases, the reciprocal rate method has proven to be robust and consistent. The primary technical contributions of this work are:Direct method to estimate reserves using only rate-time data (time, rate, and cumulative production).The reciprocal rate method is based on variable-rate theory, and is more rigorous than Arps approach (expo-nential or hyperbolic rate relations). Introduction Simply put, the Reciprocal Rate Method is an unsophisticated, yet theoretical approach for estimating reserves. The govern-ing equation is derived in Appendix A — and for convenience is given as the "Arps" form of the result: [Arps (1942)] Equation (1) Where the qi and Di parameters can be derived from theory for the black oil case (see Appendix A). For reference, the Arps exponential model is given as: Equation (2) It is important (perhaps critical) to note that Eq. 1 (and 2) are derived under the assumption that the well is producing at a constant flowing bottomhole pressure, pwf. Our contention is that the Reciprocal Rate Method is robust and will tolerate changes in pwf, particularly smooth changes. We illustrate the robustness of this method using appropriate field examples. For convenience, we write Eq. 1 as a simple straight-line relation with arbitrary coefficients. This form is given as: Equation (3) Multiplying through Eq. 3 by the flowrate term (q) yields: Equation (4) At depletion, the flowrate will decrease to zero (i.e., q ? 0), and Eq. 4 reduces to the following identity: Equation (5) The procedure for this methodology is as follows: Step 1: Plot 1/q versus Np/q. Step 2: Estimate the slope of the straight-line portion of the data trend, m. As advice, the "later" data should yield the most consistent trend. Step 3: Take the reciprocal of the slope (m) as the estimate of the reserves which will be produced at depletion (boundary-dominated flow regime) for this particular production scenario. As noted above, the single most important constraint is the assumption of the constant flowing bottomhole pressure — however; we will demonstrate the utility of this approach, even in the presence of erratic changes in the flowing bottomhole pressures.

name of conference

  • Rocky Mountain Oil & Gas Technology Symposium

published proceedings

  • All Days

author list (cited authors)

  • Blasingame, T. A., Ilk, D., & Reese, P. D

complete list of authors

  • Blasingame, Thomas Alwin||Ilk, Dilhan||Reese, Parker D

publication date

  • January 1, 2007 11:11 AM

publisher

  • SPE  Publisher