1D Geomechanical Modelling of a Complex Naturally Fractured Volcanoclastic Reservoir, Republic of Georgia

Abstract

The objectives of this study were to deliver a pre-drill and real-time (RT) geomechanical model and wellbore stability analysis for the planned horizontal well within license Block XIb, Republic of Georgia. The main target is fractured tight volcanoclastic Middle Eocene (ME) formation. Pre-drill and RT Wellbore stability analyses were performed enabling safe mud weight requirements and mud weight sensitivity to inclination for the planned wellbore, as this area is significantly understudied in terms of rock properties, pore pressure behaviour and geomechanics. The model study was based on the drilling experience of the offset well, drilled a mile away and containing many data sets: wireline logs and borehole images, FIT/LOT, pressure measurements, drilling experience and cuttings, well construction and from the current well containing basic LWD gamma ray and mud log. The main problem areas were defined based on the model. Pore pressure drove many of the observed challenges, including the Maikop overpressured shales forming significant breakout zones, and the overpressured Upper Eocene sand and reactive Navtlugi shales zone experiencing many tight hole events in the offset well. Pore Pressure was later updated for the current well based on the drilling exponent (Dxc) calibrated with mud gas data as a part of RT Geomechanics study. The natural fracture behaviour of ME was carefully studied to identify potentially critically stressed fractures and near-wellbore fracture slip. The models examined breakout during underbalanced drilling as well as optimal well azimuths to minimize potential fluid losses in open fractures during drilling and avoid water cut during production. The study found that the originally planned mud weight was too risky and has to be increased in the overburden formations to avoid massive breakouts, as experienced in the offset well. While crossing target ME fractured volcanoclastic slightly underbalanced drilling may be possible. The pre-drill fracture stability study successfully confirmed its reliability during operations and allowed confidently make RT decisions. As a result, concern for losses lowered while moving the azimuth from Shmin to SHmax direction and mud weight (MW) could be raised confidently up to required level. The conducted studies, despite many challenges and data uncertainties, significantly clarified potential drilling risks within the license block area, which was understudied in terms of geomechanics in past years. Additional value was provided to future drilling programs as well as highlighting data gaps and pathways for further geomechanical model improvement and uncertainty mitigation. The model is the first valuable step in developing regional geomechanical understanding. Increased MW helped to avoid major tight hole events, detailed natural fractures analysis helped to select wellbore azimuth optimal to avoid fluid losses. As a result, rate of penetration (ROP) increased 2.3 times compared to previously drilled wells and the well became the first in the field history drilled with no fluid and cement losses. Pre-drill geomechanical model helped to develop the program suitable for safe drilling and later its success was proven through RT geomechanics support. RT geomechanical model update together with caving analysis demonstrated how it plays key role together with pre-drill geomechanical modelling in the successfully well delivery.