A New Straight-Line Analysis Method for Estimating Fracture/Reservoir Properties Based on Dynamic Fluid-in-Place Calculations
Authors: Clarkson Christopher R. , Yuan Bin , Zhang Zhenzihao 
Published: September 2019

Straight-line analysis (SLA) methods, which are a sub-group of model-based techniques used for rate-transient analysis (RTA), have proven to be immensely useful for evaluating unconventional reservoirs. Transient data can be analyzed using SLA methods to extract reservoir/hydraulic fracture information, while boundary-dominated flow data can be interpreted for fluid-in-place estimates. Because transient flow periods may be extensive, it is also advantageous to evaluate the volume of hydrocarbons-in-place contacted over time to assist with reserves assessment. The new SLA method introduced herein enables reservoir/fracture properties and contacted fluid-in-place (CFIP) to be estimated from the same plot, which is an advantage over traditional SLA techniques.

The new SLA method utilizes the Agarwal (2010) approach for CFIP estimation, extended to variable rate/pressure data for low permeability (unconventional) reservoirs. A log-log plot of CFIP versus material balance time (for liquids) or material balance pseudo-time (for gas) is created, which typically exhibits power-law behavior during transient flow, and reaches a constant value (original fluid-in-place, OFIP) during boundary-dominated flow. Although CFIP calculations do not assume a flow geometry, the SLA method requires this to extract reservoir/fracture information. Herein, transient linear flow is assumed, and used for the SLA method derivation, which allows the linear flow parameter (LFP) to be extracted from the y-intercept (at material balance time or material balance pseudo-time= 1 day) of a straight-line fit through transient data. OFIP can also be obtained from the stabilization level of the CFIP plot.

Validation of the new SLA method for an undersaturated oil case is performed through application to synthetic data generated with an analytical model. Thenew SLA results in estimates of LFP and OFIP that are in excellent agreement with model input (within 2%). Further, the results are consistent with the traditional SLA methods used to estimate LFP(e.g. the square-root of time plot) and OFIP (e.g. the flowing material balance plot).

Practical application of the new SLA method is demonstrated using field cases and experimental data. Field cases studied include online oil production from a multi-fractured horizontal well (MFHW) completed in a tight oil reservoir, and flowback water production from a second MFHW, also completed in a tight oil reservoir. Experimental (gas) data generated using a recently-introduced RTA core analysis technique, were also analyzed using the new SLA method. In all cases, the new SLA method results are in excellent agreement with traditional SLA methods.

The new SLA method introduced herein is an easy-to-apply, fully-analytical RTA technique that can be used for both reservoir/fracture characterization and hydrocarbons-in-place assessment. This method should provide important, complementary information to traditionally-used methods, such as square-root of time and flowing material balance plots, which are commonly used by reservoir engineers for evaluating unconventional reservoirs.