Log Interpretation Problems and Their Partial Solutions in Horizontal Wells Drilled Offshore-Neutral Zone-between Kuwait and Saudi Arabia
Authors: Ogawa Naoki , Minh C. Cao 
Published: December 2007

Abstract
Formation Evaluation in Horizontal wells drilled offshore Khafji poses two acute problems.
First, the cyclic grooves on the borehole wall created by the drilling process affect adversely the Density-Neutron logs. The use of oil-based mud for wellbore stability control amplifies the adverse effect because of the added hydrocarbon effect. Typical Density-Neutron logs in the field shows large cyclic variations which, without proper corrective actions, make the determination of porosity very difficult if not impossible.
Second, the well trajectory seldom stays in the target reservoirs, but goes in and out of the sands and shales layers. Because of the change in the geometry of the wellbore with respect to the formation layers, interpreting Rt from Resistivity logs can be very challenging. Often, the deepest resistivity reading is affected by the surrounding shales, while the shallowest resistivity is affected by invasion. Essentially, without the knowledge of Porosity and Rt, no water saturation calculation is possible and this complicates the decision where to perforate and produce the reservoirs. We present some typical examples and the techniques that are used locally to overcome these basic formation evaluation problems. For the first problem, a fast and effective filtering algorithm is used to retrieve the valid information from the Density-Neutron logs. The typical runtime of the algorithm is in the order of 1 minute per 2000 feet of logs on a desktop PC. Details of the algorithm are given in the paper. For the second problem, we use the multi-depth of investigation resistivity curves from the Array Induction Tool (AIT*) to derive the best possible Rt for use in water saturation calculation. We show that the multi-depth of investigation resistivity curves are necessary to resolve the problem of proximity of beds with large resistivity contrast, which is the case when the well was drilled through a high resistivity pay zone sandwiched between a low resistivity shale above and a low resistivity water zone below.

Introduction
The concession area of Arabian Oil Company (AOC) is located offshore of the Divided Zone between Saudi Arabia and Kuwait. The main producers are the sandstone reservoirs of the Middle Cretaceous Burgan formation. AOC used horizontal completion to produce the Burgan formation from four new wells and six workover wells by the end of 1995. The purpose of the horizontal completion is to avoid water production by drilling water-free reservoirs selectively.
Oil-based mud is used during drilling to control wellbore stability. The properties of the oil-based mud are as follows: Weight 9.5–10.0 ppg, Salinity 180 ppk Chloride, and Water 80% volume. The invasion of the light oil filtrate causes the Density-Neutron logs to read low. The low Density reading is made worse (lower) by the oil-filled caves created by drilling. The typical logs oscillate wildly, sometimes as large as 30 p.u from peak to peak.
The problem of cyclic borehole grooves or rugosity has been discussed before and several techniques have been proposed. The same references also explain the origins the cyclic borehole rugosity. In this paper, we propose a simple, fast, efficient and robust algorithm that could be implemented at the wellsite
The problem of Resistivity log in horizontal has been known for a long time. Since our wells are drilled with oil-based mud, the usage of a Laterolog device is not appropriate. This restricts the choice to either a conventional dual-depth of investigation Induction tool, or a multi-depth of investigation Array Induction Tool (AIT*). The latter has been discussed in details. The study of Induction responses in deviated wells is a complex subject because of the intervention of numerous effects such as invasion, dips, anisotropy, and proximity on the measured resistivity.

Porosity Determination
Fig. 1 shows a typical Density-Neutron log in an horizontal well in Khafji.
P. 179^