Drainholes have been drilled in several areas of the world and there is a need to understand these drainage systems for accurate well test analysis. This study presents an analytical solution for the transient pressure response of a uniform flux horizontal drainhole in an anisotropic reservoir of finite thickness. The solution also applies for a reservoir with multiple drainholes in a vertical array. The solution shows that there are two possible types of transient pressure behavior depending on the length of the drainhole relative to the height of the reservoir. If the drainhole is short, initial radial flow perpendicular to the drainhole axis is followed by a transition to a perpendicular to the drainhole axis is followed by a transition to a pseudo-radial flow period. If the drainhole length is long relative to pseudo-radial flow period. If the drainhole length is long relative to the reservoir height, the initial radial flow period ends rapidly and transient pressure behavior becomes identical to that of a uniform flux vertical fracture.
The pressure transient response for multiple drainholes is identical to the single drainhole solution if dimensionless variables are defined relative to the number of drainholes. Consequently, the pressure response of a uniform flux vertical fracture can also be pressure response of a uniform flux vertical fracture can also be approximated by a vertical array of drainholes. The pressure response for infinite conductivity drainholes is also suggested by analogy to the infinite conductivity vertical fracture solution.
Log-log type curves are presented for various drainhole radii and can be used in the conventional manner to determine reservoir characteristics including directional permeability or drainhole half length. Short and long time approximations are presented along with appropriate time limits. Finally, conditions for greater productivity than with vertical wells or hydraulic fractures are presented.
Drainholes are horizontal bores drilled from a vertical wellbore to increase production. The purpose of this study is to present a means for understanding these systems using, pressure transient analysis. Recently, many applications of horizontal drainholes have been attempted or suggested. This increased popularity is due mainly to attractive economics and better drilling technology.' Applications for drainholes includes: productivity index increases, injectivity index increases, greater sweep efficiencies by line flooding from one drainhole to another, water and gas coning reductions, penetration of narrowly missed target zones, intersection of naturally penetration of narrowly missed target zones, intersection of naturally occurring vertical fractures and reduced wellbore damage.
The available drainhole literature can be categorized by areas of operational feasibility, productivity increases, and EOR applications. The operational feasibility literature documents the development of flexible drill collars with better directional drilling. These technological advances have reduced the amount of risk involved in drilling horizontal drainholes. Areas of the world where horizontal bores have been drilled with positive results include California Texas, France, Italy and the USSR.
The first studies on drainhole productivity using potentiometric models were presented in the mid-1650's by Perrine, Roemershauser and Hawkins, and Landrum and Crawford. These three studies concluded that (1) drainhole length is of primary importance to productivity, (2) additional drainhole footage is more effective on a single well than are additional drainholes, and (3) increases in productivity are proportional to the combined length of all drainholes in the system until a point is reached were additional benefit per added foot begins to diminish. The conclusions of these reports are explained by the drainhole pseudo skin factor presented in this study.
Publications have been presented in the early 1980's that give analytical expressions and field results for determining expected productivity increases. The studies showed that horizontal bores productivity increases. The studies showed that horizontal bores were most advantageous for:
tight reservoirs, specially if vertical fractures are suspected
thin oil columns or a small density difference between water and oil
soft formations such as chalk that are likely to collapse
The third area of drainhole literature is EOR applications of drainholes. The primary work in this area is from Canadian production of heavy crudes. Suggested or applied uses include production of heavy crudes. Suggested or applied uses include continuous steam injection, cyclic injection production techniques, production of tight chalk formations and gravity drainage of production of tight chalk formations and gravity drainage of steam-heated oil through parallel drainholes.
Although no published studies on the pressure transient analysis of horizontal drainholes currently exist, several studies in the pressure transient analysis literature are applicable to this study. pressure transient analysis literature are applicable to this study. First is the partially-penetrating uniform flux fracture solution presented analytically by Raghavan et al. The general vertical fracture solution presented analytically for infinite conductivity and uniform flux fractures by Gringarten et al. will be used to find similarities with the horizontal drainhole pressure transient response. A third study of interest is the pressure transient response for a slanted wellbore presented by Cinco. Limiting cases of these three studies approximate the drainhole case and can be used for checking the drainhole solution.