Interpretation of Estimated Horizontal Well Lengths' From Interference Test Data
Authors: Issaka M.B. , Ambastha A.K. 
Published: March 2010

Abstract
This study uses interference test data from an analytical solution for a horizontal well in a closed reservoir to estimate the effective length of a producing horizontal well. Interference data are generated for observation points along the horizontal well, and at other points away from the horizontal well.
Effective well lengths are estimated from data corresponding. to the early radial, early linear and late pseudo-radial flow regimes for several observation points along the horizontal well. Assuming an accurate knowledge of relevant reservoir data, analysis of the well test data shows that the early radial flow dataprovide accurate, but slightly underestimated, values of horizontal well length, except when the pressure recorder is located at the end of the well. Estimates of the well length made fromthe early linear and late pseudo-radial flow data vary significantly, depending on the location of the observation point along the well.
Interference tests at observation points away from the well do not exhibit the early radial and early linear flow regimes, except when the observation point is very close to the active horizontal well. For observation points that are very close to the active horizontal well, accurate estimates of the effective horizontal well length may be obtained from horizontal well interference test data. Estimates of the effective horizontal well length are found to bemore accurate for longer horizontal wells.

Introduction
A knowledge of effective horizontal well length is important to assess formation damage around a horizontal well and to identify the productive section of the well length. Horizontal wells may be several times more productive than vertical wells, due to the larger area of contact of a horizontal well with the reservoir. Among other things, horizontal wells are particularly advantageous in thin reservoirs, reservoirs with a significant bottom water zone or gas cap, and reservoirs with natural fractures. However, the several advantages of horizontal wells over vertical wells may not be realizedf a significant portion of the horizontal well is damaged or completely plugged off. Plugging of portions of the horizontal well often' occurs due to the flow of sand into the wellbore. Production logs run in 'a horizontal well can identify what segments f the well are contributing to the production. However, an independent estimate of effective horizontal well length from a pressure transient test can assist in assessing formation damage around a horizontal well and in identifying the productive section of the well length. This study uses interference test data from an analytical solution for a horizontal well in a closed reservoir to estimate the effective length of a producing horizontal well.
Analytical solutions for the pressure behaviour of uniform-flux, as well as, infinite-conductivity horizontal wells have been discussed in the literature(1–6). Using successive integral transforms, Goode and Thambynayagam(1) presented a solution for an infinite-conductivity horizontal well located in a semi-infinite, homogeneous and anisotropic reservoir of uniform thickness and width. Ozkan et al.(2) compared the performances of horizontal wells and fully-penetrating vertical fractures. For the horizontal wellbore, both infinite-conductivity and uniform-flux boundary conditions were used.