Accurate placement of a horizontal well within a reservoir can be complicated and with uncertainties (McLennan, 2006). That was the case for the 8.5-in. horizontal well in the study being reported. Uncertainty in the structural geology existed due to distance from the closest well (~500 m) and also the vast number of faults identified on the seismic data.With the well supposedly landed in the reservoir, the expectation on start drilling sand was not met upon drilling out the casing shoe. Approximately 180 m MD of shale was encountered before making a decision to use the well for appraising the upper seismic reflector. The section was subsequently abandoned for a sidetrack that aimed at producing the upper sand lobe.From the original casing shoe of the landing point, to access the upper sand lobe with the shortest shale section possible, a strong build in inclination to >90˚ would be required upon exiting the shoe. Once the wellbore entered the reservoir sand package through the base, a change in trajectory was immediately required to avoid exiting through the top of the thin sand. The sooner the well entered the sand, the greater the success of the well because drilling more than 410 m MD would intersect the drainage radius of another producing well; hence, creating undesired production interference.A new model was developed and the well plan was executed. Based on the model, approximately 140 m MD of shale was expected before intersecting the base of the reservoir; however, in actuality, 167 m MD of shale was drilled prior to intersecting the reservoir entrance. Within 20 m MD inside of the reservoir, an indication of the top of the reservoir was observed on the distance-to-boundary inversion. As a result, the trajectory was adjusted accordingly to prevent exiting the reservoir that resulted in achieving 60% of reservoir sand.This case study will highlight how the combination of real-time distance-to-boundary mapping technology and proactive steering decisions aided in eliminating a second consecutive sidetrack of the horizontal section.

Combination of well design practices, geo-steering with neutron-density and multilayer bed boundary mapping tools with a motorized rotary-steerable service (RSS) bottom-hole assembly (BHA) has been successfully used in the Ghawar field to accurately detect multiple formation layers enabling drilling performance improvement and optimized well placement services in challenging carbonate wells. The objective of the work-over program is to establish water-free gas production from the reservoir, especially as the gas-water contact (GWC) rises with on-going production. The Ghawar field is located in the eastern part of Saudi Arabia which contains non-associated gas in the target formation varying greatly in depth from the North and South of the field. The target formation consists of major gas bearing intervals, known as Carbonate Layer A and Carbonate Layer B. The Carbonate Layer-A averages about 120 ft in gross thickness and consists primarily of dolomite capped by anhydrite. The Carbonate Layer-B formation, like the Carbonate Layer-A, consists mainly of dolomites capped by tight anhydritic dolomites. In addition, these wells are drilled in the minimum horizontal stress directional for the advantages of optimized hydraulic fractures during stimulation phase and thus improved productivity. But these wells are notorious for stuck pipe risks, tripping difficulties and slow drilling penetration rates (ROP) with high shocks and vibrations. These risks are primarily due to geo-mechanical wellbore instability and uncertainty in both GWC depth and reservoir pressures arising from the strategy of drilling through multiple layers. With very low contrast in resistivity and the complex nature of the targeted reservoir, steering with only resistivity contrast using conventional bed boundary techniques would not suffice. Ideally, steering in a single layer of the target formation will eliminate the risks associated with the traditional steering method of passing multiple layers. Neutron-density combined with a new multilayer bed boundary mapping service were successfully deployed in deep gas Udhailiyah on four different wells. This service provided precise delineation of targeted reservoir layers in addition to giving an estimate of formation dip resulting in faster and more accurate geosteering. Steering effectively in these complex thinly bedded reservoir layers has shown improved drilling and tool reliability indicators, including incremental ROP improvement, zero stuck-pipe incidents, stick-slip and shock reduction, and the confidence to push with maximum parameters with a motorized RSS BHA to minimize open hole exposure and avoid borehole deterioration effects with time.

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