Net Pay: What Is It? What Does It Do? How Do We Quantify It? How Do We Use It?
Authors: Denney Dennis 
Published: February 2015

This article, written by Senior Technology Editor Dennis Denney, contains highlights of paper SPE 123561, ’Net Pay: What Is It? What Does It Do? How Do We Quantify It? How Do We Use It?’ by Paul F. Worthington, SPE, Gaffney, Cline & Associates, prepared for the 2009 SPE Asia Pacific Oil and Gas Conference and Exhibition, Jakarta, 4-6 August. The paper has not been peer reviewed.

Knowledge of net pay is important in the volumetric estimation of hydrocarbon resources, a practice that underpins the value of the petroleum industry. Yet, there is no universal definition of net pay. Net/gross pay constitutes a major source of uncertainty in volumetric resources estimates, second only to gross rock volume. A defensible rationale to guide the quantification of net pay (thickness) is presented. Central to this process is the identification of net-pay cutoffs. 

Introduction

Net pay is a key parameter in reservoir evaluation. It identifies penetrated geological sections that have sufficient reservoir quality and interstitial hydrocarbon volume to function as producing intervals. It contributes to estimating a meaningful in-place volume against which recovery efficiency can be assessed usefully. Thus, net pay is central to the static-volumetric method of estimating ultimate recovery. Moreover, it facilitates reservoir simulation, because nonreservoir rock does not need to be characterized for inclusion.

The full-length paper details methods of quantifying net pay by use of formation properties, and it promotes a method with fewer drawbacks. These matters are especially important during the early stages of field life, when uncertainty in estimated petroleum resources is greatest.

Formation Thicknesses

The total thickness of an evaluation interval, either along hole or in true vertical space, is termed “gross thickness.” The term “net thickness” indicates that some of the gross thickness has been removed. There are three types of net thickness: net sand, net reservoir, and net pay. Their interrelationship is summarized in Fig. 1. Net pay is a subinterval of net reservoir, which is, in turn, a subinterval of net sand. These thicknesses are used along hole for completions purposes and in true vertical space for volumetric computations.

This classical terminology is rooted in the onshore oil industry. Although its use is widespread, it is inexact. Net sand would be described better as “net potential reservoir,” a term that encompasses, for example, carbonates and fractured basement. Net-reservoir intervals contain rocks that are identified as having a useful capability to store fluids and allow them to flow. In this respect, the term static volumetrics is potentially misleading. Net pay is a descriptor that originates in single-well completions onshore, where technical and economic decisions are contemporaneous. Where economic decisions are made on a field scale, a better term might be “net hydrocarbons.”

In terms of the adopted convention, the ratio of net-to-gross thickness takes three forms: net/gross sand, net/gross reservoir, and net/gross pay. It is not appropriate to use the term “net/gross” without qualifying what it is.