A Novel Multi-Probe Solution to Overcome Fluid Sampling Challenges in Low Permeability Carbonate Reservoirs in the Middle East
Authors: Telang V. N. , Sepehri S. , Mathew Renoy 
Published: February 2024

Acquiring high-quality formation pressures and representative fluid samples in low permeability reservoirs is challenging because of deep mud filtrate invasion. Additionally, high differential pressures limit flow rates of downhole sampling pumps thus requiring higher clean-up volume and stationary time. Existing technologies such as inflatable packers isolate a section of the wellbore and maximize the flow area and thereby help mitigate some of these challenges. However, this application may incur additional costs, reduces operational efficiency, and increases safety risks as inflatable packers are prone to differential sticking and undergo permanent deformation after multiple settings, causing well swabbing when pulling out of the hole.

In low permeability formations, multiple attempts may be necessary to pick the best depth for sampling. Hence, a probe-based tool having a large in-flow area is best suited to perform quick mobility checks followed by pumping out efficiently at high rates with reduced differential pressure across the probe. Deploying a probe-based tool for sampling in low mobility reservoirs significantly improves operational safety and mitigates the risk of differential sticking as compared to inflatable packers by reducing contact area with the formation and allows flow across the tool string. It also adds to maximising the operational efficiency by offering quick unlimited number of settings and providing higher flow rates through increased flow area compared to existing probe-based sampling tools.

High-resolution numerical simulation results show clear advantages of the novel probe in its operating range and sampling efficiency, compared to existing technologies. Since the flow area of the multi-probe is less than 10% of the inflatable packers, to clean-up to formation fluid, a significantly less volume of pump-out is required to clear mud filtrate from the invaded zone. A robust bottom-hole sampling assembly consisting of single probe (medium to low mobility formations), large area multi-probe and inflatable packers (low and ultra-low mobility) was deployed along with multi-sensor fluid analysers and high differential pumps. In extremely deep invaded zones, which typically require over 18hrs of pumping out using inflatable packers, the large area multi-probe tool delivered clean hydrocarbon samples by the selective slug sampling technique by pumping out 126L in 10hrs.

Multi-probe tool has the largest area open to flow among the available probe-based inlets and requires only a few minutes of setting time, making it the ideal solution for testing in low and ultra-low permeability unconventional reservoirs by reducing the dependency on high risk, cost intensive inflatable packer operations. Also, probe-based tools significantly reduce the area of contact with the formation and allow flow around the tool-string avoiding wellbore isolation as in case of inflatable packers. These key features reduce the risk of differential sticking, especially in high overbalance conditions, and allows sampling in the wells with minor static losses which is impossible using inflatable packers.