Management of geologic risk, uncertainty defines accurate horizontal well placement: addressing geological challenges prior to horizontal well executi

More than 40% of horizontal wells encounter major unexpected structural geologic/stratigraphic changes during drilling that result in wells being sidetracked or serious consideration given to this option. As the hole angle increases and measured depth of bed thickness becomes more distorted, correlation between predicted and actual geologies proportionately increases in difficulty.

Therefore, real-time correlation must be systematically planned in advance, and must identify key marker beds and decision points. Failure to recognize individual parameters within the reservoir of interest, such as stratigraphy, rock type, reservoir geometry and structure, fluid content, and formation petrophysical properties, can result in costly mistakes.
As witnessed during planning and executing horizontal drilling programs in the Hawiyah and Haradh fields of Saudi Arabia, the industry's ability to drill horizontal and high-angle boreholes has improved with the aid of proper geological planning, execution and evaluation. Advances in wireline technology adapted to the drilling environment, in combination with present measurement-while-drilling (MWD) techniques, can allow conventional logging-while drilling (LWD) tools to provide the necessary information and insight for steering boreholes geologically, and for petrophysical evaluation.
The task of assembling relevant information about the geology, as mapped, and then planning a trajectory is not a given. Understanding a reservoir's heterogeneous character and the directional relationship of reservoir permeability is required, as is the ability to adjust the well trajectory while drilling, using resistivity, gamma ray, density and porosity logs. Well planning involves delineating the targets on the basis of seismic maps, top structure maps, offset well logs, geographic target selection, and 3D modeling and visualization, to arrive at a plan of continuous well trajectory to penetrate the target reservoir zone.

Unfortunately, geologic maps are not always correct, as a result of errors in seismic processing and interpretation. Uncertainties in determining target depth and lateral position are combined with an inability to predict small-scale features, such as minor and sub-seismic faults. Therefore, real-time interpretation of all incoming data is just as important as the data, itself. Geological constraints/problems, such as offset well mis-ties, faulting, stratigraphic variations and pinch-outs, pose significant risks during drilling.

The following concerns affect accurate well placement:

* Borehole position uncertainty related to directional survey accuracy.

* Geological uncertainty (structural dip and stratigraphic), such as lateral facies variation.

* Directional control while drilling.

* Unanticipated buildup rates and doglegs found while drilling.

* Limitations of LWD tools and methods.

* Ambiguities inherent in interpreting propagation resistivity logs in horizontal wells.

Proper management of geological risk and uncertainty leads to fewer pilots being drilled with less cost.

DIVERSE RESERVOIRS, SOLUTIONS

The Haradh and Hawiyah wells are located in a geologic trend known as the Khuff. Khuff C sediments were deposited on a shelf of very low relief, where a series of repeated short-lived changes in sea level occurred, which were accompanied by exposure to meteoric fluids and dolomitization by hypersaline fluids. This produced facies variations over large areas. Khuff C carbonates are defined at the base by the underlying regional Khuff D anhydrite and at the top by extensive Khuff C anhydrite, Fig. 1. Four high-frequency sequences make up the Khuff C carbonate interval. The four sequences have been further subdivided into 12 reservoir layers (C1-12) from top to bottom.