Drilling fluid loss has been observed during drilling stage in an unconventional field. Severe or total loss of oil-based mud (OBM) is one of the main contributing factors contributing to increased drilling cost and prolonged execution cycle times. The additional cost to mitigate fluid loss can be up to more than half a million USD on single well. The impacts of lost fluid on nearby parent wells are uncertain and require more investigation on both short-term and long-term impacts. The root cause of fluid loss remains a mystery; investigation is required to discover if it is through natural cavity or fractures in reservoirs or via other routes.
Understanding the fate of lost fluids, such as where the fluid goes, whether it will flow back to the surface, and how long it takes to flow back, is critical to mitigate the impact on wells and optimize well placement and future asset development plans in the area. Oil geochemistry has been utilized to address the questions and uncertainties above. Oil samples analyzed were collected from parent wells which are above the child wells (deeper benches). Samples were collected 4-8 months after drilling fluid loss was observed. Due to the late sample collection date, impacts on produced oil from parent wells at early stage may not be captured.
The results of oil geochemistry fingerprinting study were mapped with fluid loss profiles for wells on different pads, showing contamination in the parent wells directly above the child wells that experienced severe fluid loss. Results indicate that drilling fluid was lost into the shallower formation which may migrate to the wellbore of parent wells and get "produced" together with the hydrocarbons. Oil samples collected from parent wells above the child wells which do not experience or have minor fluid loss show no significant signature of OBM contamination. The impact of lost fluid on parent wells depends upon the fluid loss volume and location. More fluid loss at shallower depth has more impacts on parent wells. Flow back of lost fluid to existing producers could last more than 8 months.
Oil geochemistry studies provide critical information to better understand the impact and fate of lost oilbased drilling fluid. Potential communication and migration path were proposed based on the results. Better sampling plan after drilling fluid loss event during drilling, fracking and Put on Production (POP) stage could lead to more accurate understanding of its impact and fate. Combining with other data such as micro seismic results, future drilling fluid loss could be avoided by selecting optimal drilling path and save millions on Capital Expenditure (CAPEX).