A Practical And Systematic Approach to Horizontal Well Test Analysis
Authors: Mattar L. , Santo M. 
Published: March 2010

The interpretation procedures for horizontal well test data that have been published in the literature to date have two principal characteristics. These are (i) they apply to drawdown data and not buildup data, and (ii) they promote segmental analysis of straight line portions of the data; when plotted using specified coordinates such as log-log, semilog, square root, etc.

The shortcomings of the segmental approach are discussed in the paper, and it is recommended that this analysis be used only as a starting point for a more comprehensive integrated model. The recommended procedure allows the interpretation of single rate and multi-rate tests (including buildups). This integrated model approach is capable of analysing data that do not form straight lines for segmental analysis. Therefore all the data are honoured, not just the straight line segments, but also the transitions between these segments.

The integrated model approach requires the use of computer Programs, and is not a manual analysis procedure. Even though the programs are sophisticated and computationally intensive, they will run efficiently on the average engineer's desktop computer.

Introduction

There appear to be two schools of thought when it comes to the interpretation of horizontal well test:The segmental analysis approach-attempts to identify and analyse individual flow regimes. for example, wellbore storage, vertical-radial flow horizontal linear flow, horizontal radial flow, reservoir boundary dominated flow.The integrated model approach-assumes a wellbore and reservoir description, generates a synthetic pressure forecast and compares it to the measured data.

Each of the above methods has advantages and disadvantages. These will be discussed below, followed by a pragmatic approach.

Segmental Analysis

Theory indicates that tor a horizontal well the following flow Regime will develop in sequence, and that from each flow regime certain reservoir characteristics can be determined:Wellbore storage will result in unit (1) slope on a log-log plot of the data and the derivative. Calculations will yield the wellbore storage constant.Vertical radial flow: On a log-log plot the derivative will have a slope of zero (0), and the data will be straight on a semilog plot (such as MDH plot). A value for permeability, k, can be calculated form this portion of the data Even though the flow is vertical permeability, but k = (√k,ky), (x is the direction of horizontal wellbore, and z the vertical direction). K2 usually significantly different from ky.Horizontal linear flow. This is identified by a slope of a half (1/2) on thr log-log plot of the derivative of a straight line on a square-root-time plot. Analysis of this flow regime yield L √(ky). If the length, L of he horizontal wellbore is known, then the horizontal permeability ky can be calculated. In some instance ky is known from othe sources (assumed equal tp ky -see discussion of next flow regime or core analysis, previouos tests or vertical well test analysis) and L can be contributing portion of effective length pf the horizontal wellbore.