Step Rate Test as a Way to Understand Well Performance in Fractured Carbonates

Abstract

Step Rate Test (SRT) is commonly used to estimate formation parting (or fracture opening) pressure for stimulated wells. SRTs may also focus on changes of well performance at different rates / pressures that is of special interest for stress-sensitive reservoirs such as fractured carbonates. Installation of permanent downhole gauges (PDG) and running SRTs on injection wells at the Ekofisk field gave a chance to improve the understanding of reservoir and well performance. Analysis of these SRTs also resulted in further development of SRT interpretation techniques. An approach to SRT interpretation, combining analytical pressure-rate (p-Q) curve analysis and step-by-step Pressure Transient Analysis (PTA) with numerical simulation of SRT, was suggested and tested. Applying this approach to Ekofisk SRTs has shown that the p-Q analysis may be used for diagnostics of well performance changes, while step-by-step PTA enables decomposing of well (skin factor) and reservoir (conductivity) effects with estimation of corresponding well-reservoir parameters as pressure or rate functions. Numerical simulation confirmed that pressure dependent conductivity estimated from the step-by-step PTA is the governing factor in matching SRT history. Special attention was paid to the uniqueness of SRT interpretation using suggested approach. Reaching infinite acting radial flow (IARF) regime at each step of a test provided unique parameter estimates as shown by example of a stimulated slanted injector. Being too far from IARF at end of each step will make the interpretations more uncertain. Different sets of changing parameters estimated from SRT interpretation could provide satisfactory match in numerical runs as was illustrated by example of a horizontal injector with multiple induced fractures. Comparison of interpretation results for different wells integrating additional field data is a possible way to reduce this uncertainty. Finally, some hints to designing, conducting and interpreting SRTs of different types of wells in fractured carbonate fields are given, using Ekofisk field experience.

PTA-metrics for time-lapse analysis of well performance

“…A multi-rate or step-rate test of a slanted fractured injection well from the study (Shchipanov et al 2017b) was chosen as the first real data set for testing of the PTA-metrics. The multi-rate test is a time-lapse PTA survey with sequential Fig.…”

“…The multi-rate test is a time-lapse PTA survey with sequential Fig. 10 Step-rate test of a slanted fractured well a and comparison of manual step-bystep PTA interpretation (dimensionless PI, kh and dP (Skin) according to the metrics ( 14)) and results of the PTA-metrics application (dimensionless WPI, RPI and CPI, according to (10)) as deviations from the reference values at step 8 b. a Also shows simulated pressure with the estimated well-reservoir parameters (Shchipanov et al 2017b) flowing periods. The well penetrates naturally fractured reservoir and was stimulated to create an induced fracture.…”

“…Permanent well and reservoir surveillance with PDG and time-lapse PTA is today employed in the oil and gas industry with focus on well performance and behavior with their changes during reservoir life-time, including fluid contact movements and recovery processes (Gringarten et al 2003), (Yaich et al 2012), (Ugoala et al 2013), (Rushatmanto et al 2017), (Suleen et al 2017), (Cassie et al 2018); well stimulations (e.g., acid and fracture jobs) and monitoring of well performance over time (Shchipanov et al 2014), (Shchipanov et al 2017a(Shchipanov et al , 2017b; and any kind of improved/ enhanced oil recovery pilots such as water shut-off (Aamodt et al 2018) and silicate injection (Skrettingland et al 2012), (Skrettingland et al 2014). Figure 1 illustrates typical timelapse pressure transient responses for sequential well shutins from (Shchipanov et al 2017a).…”

“…The difference may also help in disclosing well interference effects. Further study (Shchipanov et al 2017b) has extended the scope of time-lapse PTA to deal with multiple rate pressure transients and step-rate test analysis to monitor performance of wells with induced fractures. Aamodt et al (2018) discussed water shut-off jobs with massive cement placement into an injection well to improve water conformance control, where permanent pressure surveillance and PTA, including combined flowing/shut-in analysis, were used to identify well communication and to monitor the job efficiency.…”

Well Injectivity during CO2 Geosequestration: A Review of Hydro-Physical, Chemical, and Geomechanical Effects

“…SRTs are usually followed by a reduced rate step or well shut-in with a pressure fall-off (PFO) test . The pressure-transient analysis (PTA) of a PFO test is an inexpensive surveillance and diagnosis technique which can indicate permeability changes, induced fracture dimension, fracture closure pressure, stress skin factor, etc. , Owing to the reasonably huge number of shut-ins throughout the injection history of an injector, valuable pieces of information on the half-length ( X f ) and height ( h ) of the induced fractures can also be obtained by interpreting a PFO test .…”