Horizontal wells have been drilled in many oil and gas reservoirs. Measuring or calculating the productivity of horizontal wells has been difficult because of their long length in the formation compared with vertical wells and because of the inadequacy of measurements during drilling and production. In this paper, we examine the performance of horizontal wells. First, we review the theoretical productivity of horizontal wells and examine the sensitivity of productivity to well and reservoir parameters. We then compare theoretical and actual productivities from horizontal wells in several oil fields to show that most horizontal wells do not produce at their full potential because of the effects of wellbore and near-wellbore formation conditions right after drilling on cleanup. Second, using field examples, we show that the new integrated production logging measurements can be used to assess horizontal well productivity by identifying productive zones, fluid entries, and determining the fluid distribution (oil, gas, and water) in the wellbore. We also show how reservoir heterogeneity affects the reservoir pressure distribution. The field examples indicate that horizontal wells have permanent water sumps, regardless of whether they produce water. In these wells, we show that when a large percentage of the wellbore cross section is filled with water, reduced oil entry occurs. To assess the effect of the water sumps, flow restrictions, and noncontributing sections along the wellbore on the well productivity, a numerical model with actual well trajectories and openhole and production log measurements is used to estimate well productivity. These horizontal well performance simulations indicate 30 to 50 percent productivity reduction when they are compared with their full potential. P. 231

Traditional magnetic resonance (MR) acquisition modes tend to focus on the hydrocarbons found in high-porosity permeability rocks. This requires acquisition optimized for longer relaxation times (T1 and T2). Interest in shale resources (gas and oil) has resulted in a reexamination of MR acquisition sequences. Shale hydrocarbon resources are usually in rocks with very fast relaxation times for which traditional application schemes are not optimal.

Sixty years ofcrestal gas injection in the Mauddud zone ofthe Bahrain field has been instrumental in maintainingreservoir pressures and preventing water encroachment. Oil drainage from the gas cap has been a very successful and efficient means of oil recovery. Determining an accurate residual oil saturation in the gas cap is vital for refining reserves estimates and calculating EOR targets.

This paper was prepared for presentation at the 8th Abu Dhabi International Petroleum Exhibition and Conference held in Abu Dhabi, U.A.E., 11-14 October 1998.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractEfficiently identifying and evaluating the producibility of fractures after drilling through potential reservoirs remains a challenge to both the oilfield operator and explorationist. Timely identification of features crossing the drilled borehole, indication of feature permeability, and testing of the well at the most promising depths with the drilling rig still on location are the keys to cost-effective development of tight and fractured reservoirs.The Kurrachine Dolomite formation in central Syria is of Middle Triassic age and has low primary porosity. Economical production from Kurrachine reservoirs is therefore highly dependent on the drilled wells encountering zones of permeable secondary porosity. This paper presents a field example demonstrating the success of wireline-conveyed services in evaluating the producibility of the Kurrachine Dolomite. The services involve the latest technology sensors for petrophysical formation evaluation, electrical imaging of the borehole to evaluate the depths of electrically conductive features crossing the hole, acoustic measurements to evaluate the relative permeability of the conductive features, and testing of the depths of interest by sampling, pressure measurement, and real-time formation fluid analysis. Analysis is presented to show which wireline measurements are particularly important to the operator in this environment. The paper presents techniques successfully implemented to overcome the challenges in formation pressure evaluation and fluid sampling of the Kurrachine formation.Because the formation is accurately evaluated in real time with the drilling rig on location, the combination of services permits efficient, cost-effective decision making for development of Kurrachine Dolomite reservoirs.