LINEAR TRANSIENT FLOW SOLUTION FOR PRIMARY OIL-RECOVERY WITH INFILL AND CONVERSION TO WATER INJECTION

In this paper, we analyze the effects of primary production, producer infills and repressurization by water injection in a low-permeability, compressible, layered reservoir filled with oil, water and gas. The s ample calculations are for the California Diatomites, but the equation s apply to other tight rock systems. Primary oil recovery from rows of hydrofractured wells is described by linear transient flow of oil, wa ter and gas with the concomitant pressure decline. During primary, it may be desirable to drill infill wells to accelerate oil production. A t some later time, the infill wells may be converted into waterflood i njectors for pressure support and incremental oil recovery. We analyze the pressure response and fluid flow rates for the original wells and infill wells drilled halfway between the original wells, and - finall y - from water injection at the infill wells. All of the formation and fluid properties are described by a. single hydraulic diffusivity ass umed to be independent of time and production or injection. We solve t he one-dimensional pressure diffusion equation analytically using pres sure boundary conditions at the original and infill wells and use supe rposition to account for the water injection. We give solutions for th e pressure in the formation, oil, water and gas rates and cumulatives at both the original wells and infill wells as functions of time. Fina lly, we present a computational example of oil production from a stack of seven independent diatomite layers with different properties and s how the effects of infill wells and water injection on the total oil p roduction. We show that a single-layer analytical solution and a 1-D n umerical simulation for primary production in the diatomite agree well . Our analysis can predict the onset of pressure depletion and quantif y how long to produce from the infill wells before injecting water. We show that producing from the infill well for a few years significantl y increases the production from the field and can minimize the lost pr oduction at the infill well because of conversion to a waterflood inje ctor.