Geosteering and Drilling Challenges in a Faulted Reservoir Northern Oman
Authors: Vasquez Bautista Ramiro Oswaldo , Gonzalez Vanessa Carolina , El Hawy Ahmed , Al Zarafi Alaa , Al Busaidi Adil , Liu Ruikun , Al Shidhani Ali , Picha Mahesh , Al Rashdi Hussain 
Published: October 2018

As in most of the Sultanate of Oman fields, faulted Shuaiba fields contain formations that are extremely faulted and folded. These conditions are a result of the extensive and complex tectonic activities that broke the rock into many structurally deformed blocks. Several studies have been conducted to identify the best drilling and geosteering methods to use in the area. An additional challenge in faulted Shuaiba fields is the bounding of the target reservoir by two dense and sticky layers with similar gamma ray, resistivity, and density. With such reservoir character, differentiating between the top and bottom to make the correct geosteering decision is a real challenge when using conventional logging-while-drilling and standard drilling technologies.

A deep-directional boundary mapping tool enabled determining the borehole position inside the steeply dipping carbonate reservoir. Based on the mapping tool's directional measurements, the trajectory was adjusted to avoid exiting the reservoir from the top or bottom, thus continuously keeping the borehole within the reservoir sweet spot. A hybrid rotary steerable system (RSS) tool enabled achieving high doglegs over a short distance in response to the steep and sudden formation dip changes. If a sidetrack was found to be necessary, the hybrid RSS provided the ability to perform an openhole sidetrack in the same string to as deep as 897 m from the 7-in. liner shoe. At the same time, well design, bottomhole assembly (BHA) design and drilling parameters and envelopes were optimized, allowing new historical field records to be achieved in such challenging drilling environment, specifically, the a faulted Shuaiba fields, and in nearby Qarn Alam cluster fields.

Due to the difficulty in mapping the reservoir boundary in faulted Shuaiba fields, the operator's geological model was determined to be insufficient. With the high-resistivity contrast in faulted Shuaiba fields, the deep-directional boundary mapping tool enabled the geosteering engineer to detect the top and bottom of the reservoir to a distance up to 2.5-m true vertical depth (TVD). The ability to detect the top and bottom of the reservoir provided reasonable time to react to any sudden changes in the formation. Introducing the directional boundary mapping tool made it possible to update the geological model based on the data obtained from the tool.

During the prejob modeling, the well placement team, drilling team, and the operator's reservoir management team jointly set the geosteering objectives and assessed the risk of sidetracking the well, selected the appropriate BHA, and determined if the well would be drilled in the flank zone area. Drilling in the flank zone area was important due to the highly faulted area and sudden formation dip changes.

Due to having a better understanding of the true vertical depth (TVD) and azimuth of the faulted Shuaiba reservoirs and being able to update the structural model based on the results and boundary mapping after drilling each well, the number of required sidetracks decreased. The hybrid RSS tool enabled the well placement team to make the quick changes in the trajectory needed to avoid the reservoir top or bottom. When the sidetrack was needed, the sidetrack point could be at any position of the trajectory due to the hybrid RSS tool's capability.