Horizontal Injection and Production Wells for EOR or Waterflooding

This paper describes the potential benefits of using combinations of horizontal injection and production wells for EOR processes or waterflooding. Our results show that a very favorable configuration occurs when two opposed horizontal wells are drilled from injection and production wells so that the opposed laterals are parallel in the patterns, and extended until the horizontal segments almost meet midway between the like wells. Compared to five-spot patterns with vertical wells, opposed horizontal wells can increase injectivity (injection rate per applied pressure drop) by as much as a factor of ten, depending on well spacing and formation thickness. Areal sweep efficiency can be increased by 25% to 40%. The horizontal-well advantages are greatest for thin formations with wide spacing, and decline significantly for thick formations and/or close spacing. Also, for a given injection pressure, the pressure gradient in the bulk of the reservoir can average several times greater when using opposed horizontal wells than when using vertical wells. This could significantly improve microscopic displacement efficiencies for EOR processes, such as micellar/polymer flooding, that are sensitive to interfacial tensions. Because of the better sweep efficiencies, faster flooding rates, and/or lower injection pressures that are possible with horizontal wells, all EOR methods should benefit by their use. For example, polymer floods can be improved by the higher injectivity and lower rates of shear at the injection sandface. The advantages of horizontal wells for C02 flooding include: (a) delayed C02 breakthrough because of the better sweep efficiency, @) the potential for maintaining the MMP in more of the reservoir with no increase in the injection pressure, (c) better injectivity at the same pressures, and (d) the opportunity to convert more pumped producers to flowing wells. Thermal EOR was not investigated in this work, but the cited references show that horizontal wells have been successful in several field applications and more projects are being planned. INTRODUCTION The use of horizontal wells has been increasing very rapidly throughout the oil industry as advances in drilling techniques continue. In many reservoirs, horizontal wells can help solve a number of oil production problems. However, in spite of a tremendous increase in literature references, little information is available on horizontal-well applications for EOR methods. A survey of the extensive horizontal-well literature is beyond the scope of this paper, but reviews,lS book^,^-'^ SPE Reprint ~e r i e s , "~ '~ and published articles (over one thousand, even excluding newspaper articles) show that horizontal wells are still used primarily in problem reservoirs or to solve specific production problems. These include: lowpermeability formations especially fractured formations such as the Austin chalk,"-" low-permeability as r e~e rvo i r s ,~~ B,= unusual as sources such as coal-bed methane or Devonian ? as or water thin and viscous oil. 2,33 Most of the EOR activity has been in the area of thermal recove%-39 primarily for steam stimulation and steam drives, where both horizontal injection wells and production wells have been tried. A potential for gas EOR projects with horizontal wells is indicated by recent simulation and model studies for inert gas40 and C02 injecti~n. ~' The interest in horizontal-well waterflooding is very recent with most reports or publications appearing in 1991 and 1 9 9 2 . ~ ~ " ~ Although there is relatively little published information on the use of horizontal jniection wells, other than for thermal recovery, the need for patterns of both horizontal injection and production wells, or opposed vertical fractures, to increase the rate of flooding in EOR processes has been menti~ned. ~~-~O This paper describes the improvements in sweep efficiency and flooding rates that are possible if horizontal wells are used for waterflooding or for any EOR method which requires the use of both injection and production wells. In addition, the potential for increased microscopic displacement efficiency at the faster rates (with no increase in well-head pressure) is examined for some of the EOR methods. 2 HORIZONTAL INJECTION AND PRODUCTION WELLS FOR EOR OR WATERFLOODING SPE 23952 INCREASED INJECTION AND PRODUCTION RATES FOR HORIZONTAL-WELL PATTERNS A relationship between the rate of flooding and the amount of oil displaced by surfactant solutions was observed many years ago.51 In general, faster displacement rates increased the oil recovery for almost all laboratory experiments with surface active agents. Although later work showed that there is a specific minimum rate for the displacement of oil with surfactants, floodin at rates above this critical value always produced more oil! 8p52 Therefore, different possibilities for increasing the rate of flooding (or the average pressure gradient) for surfactant floods in the field were examined. A major obstacle to efficient oil recovery was the inefficient radial flow between injection wells and production wells. As early as 1933, studies showed that the available pressure gradient for about 75 2 of a waterflood pattern was only % of the average pressure gradient.53 To avoid high pressure losses from radial flow near wells, other geometric arrangements were considered. The two best possibilities appeared to be opposed lateral drainholes (now usually termed "horizontal wells") and opposed vertical fractures. Because lateral drainholes had not been drilled more than 100 feet in the fifties (although Eastman claimed that they could be extended to greater distance^),'^ and the costs were much greater than vertical wells, the parallel and opposed vertical fractures (to create a linear flood) were given more attention.47 Maximum Flooding Rates for Linear Floods Before discussing the benefits of horizontal injection wells, it is instructive to examine the difference between the flooding rates in a normal five-spot pattern and the thearetical rate that could be achieved by a full linear flood (possibly with vertical fractures) with the same spacing. The steady-state rate of water injection in a five-spot pattern is given by:'