Based on the relationship between formation pressure and minimum horizontal stress, as well as the low leak-off rate and abnormal high reservoir pressure commonly observed in tight sand, the authors devised a new method to calculate the reservoir pressure in tight sand using the pre-frac test. Numerous methods have been published to calculate reservoir pressure using pre-frac test. However, all of them require extended pre-frac testing time, taking hours in conventional formation while days in tight sand. It is not practically viable to keep the frac crew waiting so long before a frac can be executed. In contrast, this new method uses the information collected from a 5-minute pre-frac test, which is standard routine for fracs in the Denver-Julesburg basin (DJ basin). Furthermore, this new method has been validated by the conventional well tests conducted by several service companies.
The Codell Formation is the primary reservoir at the Wattenberg field in the DJ basin. Due to the nature of its offshore depositional origin, the reservoir quality of the Codell sandstone has been deteriorated by high shale content and intensive bioturbation, as well as diagenesis. As a typical basin center reservoir, the Codell sandstone at the Wattenberg field possesses abnormally high original reservoir pressure, which is one of the major driving forces of well performance. Because of extended development history and intensive drilling activity, reservoir pressure depletion has been reported in many areas. It has been confirmed that well performance can be substantially affected by pressure depletion, which makes this already economically marginal formation more economically risky. Therefore, the success of infill drills largely depends on the current distribution of the reservoir pressure.
Two types of commercially available well-testing methods have been applied at the Wattenberg field. One is the Pressure Express (XPT) developed by Schlumberger, and another one is the Dynamic Injection Fracturing Test (DFIT) performed by hydraulic fracturing companies. We conducted the XPT in several wells. It is the first time to utilize this type of tool in the Wattenberg field. Because the XPT test requires extended testing period in order to reach the radial flow regime in tight sand, tool stuck in open hole is always a concern. In many cases, tests had to be terminated prematurely for this issue. On matter of the DFIT tests, it usually takes two days to complete a test at one depth. In order to obtain multiple pressures in one wellbore, several days is needed before the well can be hydraulically fractured and put into production.
Using pre-frac tests to calculate reservoir pressure in tight sand in the Rocky Mountain region has been published (Rajappa, 2007). This method employed the same theory as the DFIT test. Just like the DFIT test, using the pre-frac test to calculate reservoir pressure requires extended waiting time to ensure that the test reaches the radial flow regime. In order to accommodate the pre-frac test into a frac job, the frac crew usually moves in and starts the pre-frac test on Friday. The pre-frac lasts the entire weekend. The frac crew comes back on Monday to finish the frac job.