FMB and Rate Transient Analysis (RTA)

Introduction

The word Transient refers to the unsteady-state nature of fluid flow, and is defined as the fluid flowing condition at which the rate of change of pressure respect to time (at any position) is the reservoir is not constant, and greater than zero. In other words, not steady or pseudo-steady-state.

Rate Transient Analysis (or RTA) is a method for performance-based reservoir characterization during transient production period. According to Blasingame, these are model-based diagnostics for provide estimates of dynamic formation properties such as permeability, skin, fracture half-length, dual porosity/ natural fractures, and more - which of course can be used to provide a physics-based forecast.

RTA has its foundations in Pressure Transient Analysis (PTA). In Summary, RTA can provide:

• Reserves - this is an estimate of the recoverable hydrocarbons, and is usually determined by traditional methods like decline (in addition to RTA methods).

In RTA, the calculated volumes may be referred to as “Connected Volume“. There is a difference between the petrophysically mapped in-place volumes (the grey box) and the volume resolved by RTA (blue contacted volume)

• Reservoir Characteristics - Permeability, well efficiency, and some reservoir characteristics can be obtained from RTA methods, which are an extension of PTA.

• Oil / Gas-in-Place - The modern methods of production data analysis give OOIP and OGIP as opposed to regular EUR.

• Production Forecasting - Calibrating models for rate and/or pressure prediction.

• Completion effectiveness & frac optimization

• Drawdown management and related production optimization

The figure below, provided by Clarkson [2013] illustrates the difference between transient flow, and pseudo-steady-state. The simulated pressures were done using a vertical well in dry gas homogeneous reservoir, producing against a constant flowing pressure. The derivative, shown to the right, was calculated using the Bourdet (Well Testing) derivative.

The upper left side of the image illustrates pressures in cross-section, while the lower left side shows isobars in plan view. During transient flow, shown in green dashed curves, the pressure propagates radially away from the well - it is this time period that one can interpret permeability and skin. During boundary dominated flow (red solid lines), the pressure drops at the same rate everywhere in the reservoir, and material-balance-like calculations can be performed to interpret the wellbore pressure drop for hydrocarbon-in-place.

RTA vs PTA

Both PTA and RTA have their own limitations and assumptions in analysis. PTA involves the analysis of pressure data of few hours or days, while RTA involve the analysis of production rate data of few months or years.

PTA could give detailed information about the reservoir properties near the wellbore (whatever region was investigated during this period). Test durations are in the range of hours to days and weeks.

On the other hand, RTA could only give an average information of the reservoir properties over a certain period of time and potentially better estimates resource size. Test durations are always increasing from weeks to months, and years.

In concept:

The graphic below gives an visual representation of PTA vs RTA. In this example, there are four (4) major shut-in periods which are used for PTA work, while the remainder of ALL “flowing “data is used for RTA. In summary:

• In PTA, we focus on individual shut-in periods

• In RTA, we focus on ALL the flowing data, while ignoring ALL the shut-ins

See Also:

References

• RTA in whitson+, Mathias Carlsen, Mohamad Dahouk, Curtis Whitson, 26 June 2023