In a typical waterflooding philosophy, water injection is often constrained by the in-situ stress of the overlying geological seal, in order to avoid out-of-zone injection and ensure reservoir fluid containment. Direct measurement of in-situ stress from wireline Micro-frac testing is insightful as compared to a conventional leak-off test (LOT) conducted at the casing shoe, due to the smaller volume and lower injection rate used. Unlike the conventional LOT, Micro-frac tests can be performed at discrete and multiple intervals in one wireline run, away from the casing shoe.
In 2017, Shell Malaysia Exploration and Production (SMEP) conducted the first wireline Micro-frac test in a caprock shale, in a deepwater injector well, offshore Malaysia. The job was successfully executed with an improved design of the flowback mechanism, using a combination of a small volume pump and a drawdown chamber. While the small volume pump provided the advantage of accurate volume/rate measurement, the modified drawdown chamber setup provided a constant flow rate, to aid in a more accurate fracture closure pressure interpretation. The improved setup was the first successful application of this device for the purpose of Micro-frac. Both the methodologies showed good and comparable results, thereby adding higher confidence to the interpretation of fracture closure pressure.
A workflow has been developed to ensure successful execution of the logging job. Pre-job modeling was completed using offset well data to derive formation mechanical properties as well as in-situ stress profile. This information was then used to characterize formation breakdown pressure, selection of test intervals and optimum straddle packer positioning. Learnings from the test conducted at the first depth station were used to optimize the number of cycles for the subsequent stations. In addition, an operational decision tree was developed to assess the number of cycles required and contingency criteria.
The paper presents an improved flowback design using the drawdown chamber as well as the operational challenges in a deepwater environment. The methodology provided a reliable estimate of fracture closure pressure in a non-permeable formation. Results have been used to update the water injection envelope, in an effort to avoid undesired out-of-zone injection.
