A New Method to Evaluate Leak-Off Tests in Shallow Marine Sediments (SMS)

Abstract

Leak-off Tests (LOT) and Pressure Integrity Tests (PIT), are verification methods to evaluate cement jobs and estimate the formation fracture pressure at the casing shoe. Although analysis of LOT in deep wells is considered a well known and practical procedure, the interpretation of LOT in Shallow Marine Sediments (SMS) is less clear. LOT in SMS gives a non-linear relationship between the pumped volume and the observed pump pressure and there is no obvious point of deflection or "leak off" from the initial linear section of the plot. Because of non-linearity of LOT plots in SMS, LOT could be inconclusive and difficult to interpret. Incorrect interpretation of LOT would lead to costly problems that might jeopardize well progress such as; unnecessary squeeze jobs, premature setting of casing, and lost circulation problems. This paper presents a new method to analyze casing shoe leak-off pressure in the SMS. A mathematical model is derived to assist in analyzing non-linear LOT behavior and the new method is supported by field data from LOT in SMS. LOT interpretation is the basis for the critical drilling decisions such as; casing setting depths, cement job evaluation, drilling fluid selections, and well control methods. Correct interpretation of the LOT can help to avoid variety of problems and save considerable amount of time and money. Introduction Shallow water flow, gas migration, cratering, and underground blowouts can be prevented only through appropriate casing design and cementing jobs. The LOT is relatively simple field procedure to determine fracture gradient. Since, the LOT is a measure of the strength of the formation or the integrity of the cement at the casing shoe. Leak-off pressure determined from the test is a measure of the maximum pressure that may be imposed at the casing shoe. Although the LOT is a simple almost mechanical procedure, the outcome of the test can be hard to interpret, particularly in formations that give nonlinear relationships between the pumped volume and the observed pump pressure. Generally, in shallow marine sediments LOT procedures are conceptually the same as for the deep sediments. The difference is that in deeper sediments the beginning of formation failure is easy to recognize, which is not the case for the shallow and soft rocks.

Review of the Leak-off Tests with a Focus on Automation and Digitalization

“…LOT could be performed in both vertical [21] and horizontal wells [42]. Significant efforts have been made to model LOT or XLOT [13,43,44], and the non-linear LOT behavior was studied [45,46]. But in the last decade, most efforts have been focused on the application of the LOT method in horizontal wells [47].…”

“…9a). While by using a log-log graph, a clear deflection point could be distinguished as the slope changes from the unit slope to the half slope [46] (Fig. 9b).…”