Technological solutions for drilling a trap intrusion section on the Srednebotuobinskoe oil gas condensate field, East Siberia

Abstract

One of the largest trap intrusion provinces, Siberian traps, is located in the East-Siberian Platform in Russia. A trap intrusion zone usually has abnormally low reservoir pressures and natural fractures. Consequently, trap drilling is associated with fluid losses that can be catastrophic. The section of trap intrusion is a part of a Ø 174 mm production casing section of the Srednebotuobinskoe field. The basic well design of this section also includes the Osinsky horizon, which features an abnormally high reservoir pressure zone. The latter creates incompatible with drilling conditions environment due to the presence of catastrophic loss zones in traps. Time spent on drilling a trap intrusion zone accounts for up to 30 % of the total well drilling time. The abovementioned geological issues in directional wells become the key topic in solving it. The method to resolve this issue is an integrated approach employing all the technologies and technical facilities aimed at finding a technological solution. First of all, in order to optimize the well construction cycles and reduce the complications, all the wells were classified in three categories as per the type of behavior in trap intrusion. This allowed to work out multi-level activities, that depended on severity of losses and non-operational time spent on drilling through this section. An alternative well design was developed and trialed on several wells, which showed positive results on decreasing non-productive timing. The main concept of this design was significantly different from the basic well design that was used in the past on the Srednebotuobinskoe field. The main difference was isolation of traps from the high pressure Osinsky horizon lower section with a Ø 245 mm casing string, which allowed safe splitting of two incompatible drilling zones. The economic effect of this solution allowed saving of up to 10.4 days or 15 % of the construction time in the wells of first category. These results were reviewed at the Technical Committee of the Company and agreed to implement the alternative well design on first category wells on the Srednebotuobinskoe field. In addition, for the rest of well categories the Drilling Team has produced and successfully implemented the preventative measures that allowed drilling through traps with lost circulation material. It is worth to mention that this method assumed a by-passing mud-cleaning system on the rig to allow building up a solid phase thus stemming the losses while drilling. For all the categories of the wells the Drilling team has selected and trialed different types and designs of drilling bits that would allow drilling hard rock such as dolerite section in traps with minimal number of runs. For the last five years the Drilling team together with the bit producing companies have designed a new type of PDC cutters that would allow to enhance durability as well as improve drilling speed both in dolerite formations and in overlying formations in the Production casing section. The Drilling team has also managed to reduce the number of trips associated with early bit wear decreasing them from five to two runs and create an experimental basis for drilling the entire section of the production casing in one run. From 2019 team continue looking for further solutions that would allow to improve bottomhole assembly elements balancing between durability and drilling rate in different types of formations above and below traps. A range of proposed technological solutions significantly reduced the impact on the well construction performance by preventing and reducing the downhole losses events in the section of trap intrusion on the Srednebotuobinskoe field.