Reducing Risk by Using a Unique Oil-Based Drilling Fluid for an Offshore Casing Directional Drilling Operation

Abstract

The conventional drilling approach of employing a drillstring and bottomhole assembly with bit for drilling purposes and eventually running a casing string includes several tripping operations that leave the wellbore open for extended periods of time as an alternative to drilling-with-casing operations where the bottomhole assembly is attached to the casing. This eliminates additional drillstring tripping as drilling and running casing are performed simultaneously. This type of application reduces the number of string trips needed to complete a section, thus saving operational time and associated costs. As the diameter of a casing is larger than a drillstring, this method generally increases the Equivalent Circulating Density (ECD) as pressure losses increase owing to the reduction of the open hole and casing string annulus area. Casing directional drilling operations provide a much narrower drilling window with regards to restrictions in the annulus thus increasing the chances of reaching fracture pressure compared with conventional drillstring operations. To negate the resultant increase in ECD, the rheological profile of the drilling fluid must be designed appropriately. This paper discusses a unique oil-based drilling fluid system weighted with treated micronized barite slurry (TMBS) that has more recently been developed and used successfully in the Eldfisk field of the Greater Ekofisk area, Norway. The drilling fluid system provides low viscosity and a relatively low flat rheology, reduced torque values, and superior sag stability, thus delivering a fluid of low ECD contribution, low-pressure peaks, very effective hydraulics performance and static stability. These exceptional fluid characteristics make the system an excellent solution for drilling sections where the difference between pore pressure and fracture pressure is narrow. This was particularly so in this case where the stability of the fluid from sag potential was crucial for the improved success of the drilling-with-casing operation. Any solids sag onto the latching tools or bottomhole assembly (BHA) with the casing could have caused interference. The system had been used before for differing hole conditions including managed-pressure drilling and extended-reach wells, in high-temperature, high-pressure (HTHP) wells, and now with competence in a casing directional drilling operation. Introduction With this being the first offshore directional drilling work there is limited experience to draw from. To support the deviated drilling with casing operations comprehensive engineering studies were performed to qualify acceptable drilling parameters. This Eldfisk well on the 2/7B platform was the first offshore deviated drilling-with-casing operation ever performed (Fig. 1). The wellbore inclination was increased to a maximum of 69 degrees at the total depth of the second section while drilling with casing. The main purpose of drilling with casing operations is that drilling and casing running operations are conducted simultaneously. By operating in this manner the wellbore is secured at all times from less significant instabilities, and the drillstring tripping time and pipe handling time is dramatically reduced. This is because the BHA is being pulled on wireline for reconfiguration or bit change. The BHA latches into a purposefully designed sleeve in the casing shoe joint. When drilling with casing, the casing is rotated slowly, that is with a rotation per minute rate of less than 30. This rotation assists by "smearing" the drill cuttings into the formation of the wellbore. It helps provide an effective barrier that has been known to prevent the loss of drilling fluid into the formation. This can be quite contrary to conventional drilling operations with a drillstring where the drillstring can aggressively bounce at the wall resulting in the removal of the filter cake in permeable formations. As a consequence the formation could be exposed to the drilling fluid and its higher hydrostatic pressure causing fracturing or a reduction in the formation integrity.

A Significant MWD Tool Signal Strength Improvement: Another Advantage of Using a Treated Micronized Barite Drilling Fluid System

“…Surge and swab pressures that exceed the formation fracture pressure can lead to loss of drilling fluid into the formation and potential loss of well control. A number of papers have been written showing the benefits of TMB on drilling fluid and drilling operations (Kågeson-Loe et al 2007;Matlock and Conn 2007;James et al 2008). The following table summarizes some of the benefits of TMB.…”