Pressure Normalized Decline Analysis

Introduction

In this context, pressure normalized decline is simply rate divided by (P_i-P_wf). This is suggested by John Lee [2013] as the preferred method if BHP or WHP is avaliable (if not full blown FMB and Rate Transient Analysis (RTA))

In other words, Arp's Decline can be re-cast as:

The above can also be applied to other models discussed in Decline for Unconventional Reservoirs (refer to Collins [2016]) for additional discussion.

Although similar in concept, this is a simplification than the normalized rate and pressure variables used in FMB and Rate Transient Analysis (RTA) and Transient Models & Flow Regimes - both which take into account fluid and rock properties, in addition to flowing pressure. It is an attempt to provide a “constant pressure” rate response so decline curve analysis can be performed.

It is worth to note that the pressure drop normalization serves only as an approximation, and it is not exact

Practical Advantages of Pressure Normalized Decline

For demonstration, a decline was simulated for a gas well within a homogeneous bounded reservoir (and included some BHP pressure changes). Some of the more common decline plots are shown below. With each bottomhole pressure change, we see a corresponding rate change. This may pose difficulty for automated decline analysis if rates are changing often due to BHP changes.

Using a normalized decline is shown below. On many of the plots, the magnitude of the rate change is reduced - the plots become smoother! The greatest impact was seen on the two (2) left plots.

This is a common technique to normalize the data for operational conditions.

According to Ilk [2023], it is worth noting that pressure drop normalization serves as only an approximation in traditional decline analysis (it is not exact).

Pressure Normalized Decline & Flow Regime Identification

John Seidle [2017], a committee member of the SPEE Monograph 4, presented the following diagnostic plot as an extension of the above. Seidle [2017] also presented FMB and Rate Transient Analysis (RTA) as an important parameter diagnostic for unconventional reserves analysis. Blasingame [2015] provides a basic interpretation of Multi-Fractured Horizontal Well flow regimes with respect to some of the more common diagnostic plots

In the example below, from a Wyoming Horizontal Well 16-1H, only linear flow is observed (maybe transitional), and essentially no boundary dominated flow. For more discussion on linear flow, refer to Multi-Fractured Horizontal Well models in the FMB and Rate Transient Analysis (RTA) documentation. Predico has also developed Linear Flow Models. According to the SPEE Monograph 4, these are three (3) states to determine before applying any model.

In this additional example for Wynoming Horizontal Well Thunder Bolt 1-2H, the pressure normalized rate diagnostic plot showed substantial linear flow followed by boundary dominated flow. Seidle [2017] suggested in the examples below, you could possibly determine fracture interference. That is, Fed 16-10 Frac or Henry 14-11 Frac caused a deviation in the diagnostic plot.

John Lee [2013] presented a similar example for a horizontal well identifying both linear and boundary dominated flow.

Ortiz [2013] presented similar example where bilinear and linear flows were identified using normalized pressure vs regular Material Balance Time. In this example, Ortiz [2013] was using Pseudo Pressure

Forecasting Rates and Pressures

According to Ilk [2023], also a committee member of SPE Monograph 4, normalized rate and flowing pressure can be both forecasted to provide the “variable-pressure' response of oil rate in time.

See Also:

References:

• Dr. Dilhan Ilk, A , “Survey on Diagnostic Based Methods for Well Performance Analysis and Production Forecasting in Unconventional Reservoir”, SPEE Denver Chapter, April 12, 2023.

• John Seidle, “Adaptation of SPEE Monograph 4 to Wyoming Horizontal Plays“, October 12, 2017.

• John Lee, “Are Our Proved Shale Reserves Reasonably Certain?” Ryder Scott Conference, 2013.

• Patrick William Collins “Decline Curve Analysis for Unconventional Reservoir Systems - Variable Pressure Drop Case” Professional Studies, M.Sc. Texas A&M University, 2016.