Critically Stressed Fracture Analysis to Identify Mud Losses Mechanism While Drilling the 8 1/2" Section in Chichimene Field

While Drilling the 8 ½" section in Chichimene field of Ecopetrol S.A, mud losses were observed. This issue is explained because the equivalent circulating density (ECD) while drilling is higher than the pressure losses that acts in the reservoir zone. It has been identified that the minimum horizontal stress (?????????) in Chichimene field has decreased due to drawdown of reservoir pressure. This condition has been observed in different fields operated by Ecopetrol S.A in Colombia. Reservoir depletion along with ????????? decrease has become a problem during drilling that affect most of the fields around the world. On the other hand, natural fractures are present in the reservoir and therefore this condition has pointed out the requirement of understanding their behavior during drilling operations. It means, understanding the stress state on fractures in the drilled wells and establish if these fractures have endured any type of frictional alteration while drilling. The hypothesis of stress state alteration due to fluid invasion within the fractures planes is established in this work. In some cases, the normal effective stresses could be higher than the frictional resistance of the contact surface. This fact would generate a relative displacement between the contact surfaces. Geomechanical models for two wells were calibrated for the reservoir zones based on natural fractures and breakouts identified from image logs. According to the characteristics of natural fractures (azimuth, dip azimuth), the magnitude of normal (σ_n) and shear stress component (τ) were estimated for each plane of fracture. Then, a failure envelope with cohesion (Co) equals to zero was built and the friction angle of fracture planes were estimated as a fraction of the same parameter in the intact rock. Mohr circles were built to represent the state of stresses at fracture planes which were compared to the failure envelope. ECD measurements while drilling were used as the pore pressure in the fractures under the assumption of fluid invasion. The results showed fractures critically stressed due to high ECD values while drilling. Finally, the maximum pressure that each group of fractures can withstand before entering the critical stress state was identified in order to estimate what the authors called the reactivation gradient.