How to forecast production when there is no declining rate

Objective:

Our client was producing from a gas reservoir. For the purposes of reserves determination, and optimization of future development, the client requested that we determine the following:

• Original Gas-in-Place
• Forecast of future production (recoverable reserves)

Background:
The first three wells came on production in 2001, and an additional 3 wells in 2007 (Figure 1). The only other data supplied were:

• production rates,
• flowing wellhead pressures;
• initial reservoir pressure in 2001,
• reservoir pressure in 2007, obtained just before one of the 2007 wells was placed on production.

There was no decline in the rate. No petrophysics or geological information was provided.

Figure 1: Production history.

Methodology:

The lack of reservoir engineering and geological data was overwhelming: no logs, no cores, no porosity, no permeability, no net pay, no saturations, no area. Normally, these parameters are required starting points for any reservoir engineering study, which would encompass history matching of the production using a simulator, and subsequently forecasting different production scenarios.

Moreover, there was no decline in production rate from which recoverable reserves could be forecast.

The saving grace that enabled this study to succeed was that the daily flowing pressures were available.

The classical equations of flow describe the relationship between flowing pressure and flow rate, for a given reservoir description. In other words, the combination of flow rates and flowing pressures contains information about the reservoir, in particular, its size as well as the deliverability potential of the well. Therefore, Rate Transient Analysis has the potential to yield our objectives, namely to extract the reservoir volume and deliverability from the supplied flow rates and flowing pressures.

The IHS Markit Harmony software contains a series of reservoir engineering tools, ranging from simple Decline and Volumetrics to sophisticated Rate Transient Analysis (RTA) and Numerical Simulation. However due to the absence of information, only a few of these could be used.

1. Static Material Balance
2. Conversion of wellhead flowing pressures to bottomhole conditions
3. Flowing Material Balance (FMB)
4. Type Curve Matching (RTA)
5. Deliverability Forecasting (analytical models)

The remainder of this post has a number of equations and formulas, which are best viewed in PDF format, please read the full text of this blog for additional details.

Louis Mattar, M.Sc. P.Eng, is a Principal Reservoir Advisor at IHS Markit.
Posted 9 February 2017

This article was published by S&P Global Commodity Insights and not by S&P Global Ratings, which is a separately managed division of S&P Global.