Wellbore Strengthening and Continuous Mud Circulation Allow to Save an Expandable Liner: Field Application Offshore Italy

Abstract

Wellbore strengthening techniques have been used in recent years to increase the capability of wellbores to maintain higher pressures. By increasing the fracture resistance of formations, operators can save rig-time and large volumes of drilling fluids. The Luna-41 well, offshore Italy, intersects a critical interval comprising high pressurized formations overlaying a lower pressure depleted zone. The initial plan for the well was to divide this interval into two separate hole sections using two different mud systems. A casing string would have been set to isolate the shallower high pressure region followed by an expandable liner to isolate the over pressured shales laying above the depleted reservoir level. An alternative design was proposed that required only one fluid system and a single casing string, thus saving an expandable liner. Thanks to the wellbore strengthening application and the proprietary continuous mud circulation device, the accomplished well program allowed an 8-day rig-time reduction and a 3-MMUSD cost saving. A specific modelling tool developed for wellbore strengthening applications was used to assist with fluid design. The tool calculates the width of microfractures induced by differential pressure and the Particle Size Distribution (PSD) of carbonate materials required to plug such microfractures and ultimately strengthen the wellbore. The mud formulation for Luna-41 was tested in the laboratory using a Pore Plugging Apparatus (PPA) and aloxite discs with pore sizes corresponding to the calculated microfracture width. The fluid used to drill the critical interval was a salt saturated system based on polyglycerol complex and supplemented with a polyamine inhibitor. The field application was a success. The depleted zone was drilled without incurring lost circulation. This paper describes the results of the field application as well as the fluid engineering process and laboratory testing to highlight the benefits - such as accessing depleted reservoirs and saving casing strings - that wellbore strengthening combined with a continuous mud circulation system can bring to the industry.

Finite-Element Studies of Hoop-Stress Enhancement for Wellbore Strengthening

“…used in this paper is determined from practical experience, consistent with previous work by other investigators (Wang et al 2007a, b;Wang et al 2009). Unpublished results (Gray and Feng 2014) show only minor differences for fracture lengths of 4, 5, 6, and 7 in. For comparison of results around the wellbore wall, the location at the facture mouth is taken to be at 0 , and ranges from 0 to 90 around the quarter wellbore, as shown in Fig.…”

“…Also, this permits comparison with several publications where the horizontal-stress anisotropy of 3 has been used in analytical, numerical, and experimental studies of geomechanical problems (Hubbert and Willis 1957;Wang et al 2007a, b;Guo et al 2014). Parameter-sensitivity studies (Gray and Feng 2014) to be published include results for horizontal-stress anisotropy in more detail.…”