Gas Diagnostics: To Drill or Not to Drill?
Authors: McBurney C.J. , Baker R. , Chugh S. 
Published: March 2010

Abstract
A very important but often difficult question to answer when dealing with gas reservoirs is: to drill or not to drill? Why are some gas pools only drilled at two wells per section and some drilled up to 16 wells per section? How do you decide whether downspacing will be beneficial? An accurate assessment of important factors such as capacity (kh), original gas-in-place (OGIP), and reservoir continuity is required.
This paper describes a set of techniques that can be used to confidently assess kh, OGIP, and continuity in a gas reservoir. This information is then used to help determine if infill drilling has the potential to be beneficial. The following diagnostic techniques are simple to use and do not require a lot of time to perform. These techniques let the historical production/pressure data do the talking, so detailed geological modelling/mapping is not required. This fact, in combination with their effectiveness, makes them a must-have tool in any reservoir engineer's repertoire.
The techniques include:Using the inflow equation coupled with simple gas material balance to determine the effectiveness of recent infill wells, along with the determination of kh and OGIP;Looking at initial productivities or initial measured pressure vs. on-production data for a group of wells in the same formation to determine continuity and heterogeneity; and,Calculation of the "depletion index," which is an innovative tool in which the level of depletion is quantified, and can then be used to compare to other pools.
The techniques are described in detail and then applied in a case study where infill drilling was successful, and one in which it wasn't so successful, to illustrate how they work.

Introduction
Gas field development and optimal spacing depends upon resources in place (OGIP), the deliverability of wells (function of kh), the production profile, the reservoir continuity, and costs. Due to uncertainty in reservoir continuity, significant work has been done to optimize gas fields. Many of the previous works have used statistical techniques to identify infill potential. This paper shows a combination of analytical/statistical techniques that use an "average well" concept to determine infill drilling potential. A tie between physical parameters (such as permeability and apparent OGIP) and infill opportunities is developed. The method is especially powerful since it shows the connectivity and incremental recovery earlier than can be confidently extrapolated from decline analysis. This technique allows the operator to make development decisions about the field more rapidly.
This analytical tool calculates the theoretical gas inflow performance through coupling of the pseudo steady-state (PSS) inflow and material balance(1) equations. The PSS inflow equation is only applicable after transient effects have subsided. The time to PSS flow can be determined with the following equation:
Equation (Available In Full Paper)
It is important to note that gas rates prior to the onset of PSS are transient and, thus, are higher than the rate that would be achieved in PSS flow when depletion is occurring.