Previous Talks

Wednesday, May 13th, 2009

Improving the Interpretation of Formation Strength Tests

Eric van Oort, Shell EP Americas


Verifying pressure integrity of a casing string and the adjacent formation is an important requirement during drilling of a well. Crucial decisions on mud weight, kick tolerance, interval total depth and casing setting depth are based on the outcome of pressure integrity tests (PIT) such as leak-off tests (LOT) or formation integrity tests (FIT). Moreover, government regulations usually require that a minimum integrity of well is guaranteed before a well may be deepened.

Yet the majority of PIT tests and their interpretation currently carried out in the field can only be characterized as inadequate. Whether due to  (in)sensitivity in the hydraulic system to subtle pressure effects in wellbore, use of highly compressible synthetic or oil muds,  non-linear thermal profiles or poorly-understood formation stress and strength behavior.  As well, inadequate data capturing, (e.g. by hand-generated plots) procedures and formation strength determinations by PIT often lack quality and accuracy. These inadequacies may have a significant negative impact on the drilling operation. For instance, when mud windows are incorrectly assessed after testing, lost circulation or well control problems may ensue on wells with tight drilling margins.

In this presentation, many of the problems underlying current PIT tests and their interpretation will be highlighted and illustrated with actual field examples (such as the under-estimated effects of mud gellation and compressibility on test outcome and the discrepancy often observed between surface readings and downhole pressure-while drilling (PWD) readings obtained while testing). Moreover, it will be shown how test artifacts can be overcome to correctly assess formation strength parameters.  The aim is to contribute to raising the general standard on PIT testing, which will benefit both safety and economical well delivery. In addition, wellbore strengthening will be discussed, including the role PIT’s can play in determining which mechanism (e.g. stress cages, fracture closure stress, increased fracture propagation resistance) is actually responsible for the strength enhancement effect.

Speaker Biography

Eric van Oort is Well Planning and Business Improvement Manager for Shell E&P Americas in Houston.  After receiving his Ph.D in chemical physics in 1990, he joined Shell Research in The Hague in 1991 to work on shale stability problems and drilling fluid design. He moved to the US in 1996 as a borehole stability and fluids R&D team lead. From 2000 onwards, he has directed operations support for Shell E&P Americas in the areas of fluids, cementing and real-time operations centers.