Targeting Horizontal Wells - Efficient Oil Capture and Fracture Insights
Authors: Tank W.J. , Curran B.C. , Wadleigh E.E. 
Published: December 2007

Horizontal well targeting has been a greater challenge in massive fractured carbonates than in low productivity, poorly connected, relatively thin reservoirs. Targets in three dimensional space have been specified to both confirm the fracture interpretation and establish high efficiency oil capture. Several well examples are presented to illustrate the targeting objectives and the resulting well performance. Early horizontal drilling objectives often pushed the operator to drill as far as possible in a selected optimum direction, using the same philosophy as is used in matrix dominated reservoirs. Results presented indicate profit maximization by drilling to a specific target to intersect a fracture trend at an optimum elevation instead of concentrating on maximizing length of lateral. Intervals of rapid penetration, lost circulation, and/or bit slides, along with cutting sample compositions, provided insight for confirmation and extension of the fracture network interpretation. The width of disturbance and degree of fracturing along pre-drill interpreted highly flexed trends is valuable data for improved fracture network interpretation and computer simulation. Both the elevation and number of fracture branches encountered are significant strategic planning issues for oil recovery from unconfined oil columns in a massive carbonate system. Results from a large number of horizontals indicate the productivity increases achieved by proper targeting of laterals into major fracture features. Proper targeting of horizontal wells in fractured carbonates can maximize profit while minimizing environmental impact by limiting the total number of reservoir completions necessary to effectively develop the field.

Introduction

Horizontal wells provide a unique assessment tool for formations containing reservoirs dominated by discontinuous flow features. Massive carbonate formations are the most extreme setting for large-scale high contrast discontinuous reservoir properties. In sandstones of moderate to low quality, horizontals are typically applied to improve rate by exposing additional formation for fluid entry at high drawdown. In carbonates, horizontals serve to intersect high conductivity features. In sandstones high flow quality often coincides with high hydrocarbon storage. In contrast, carbonate flow parameters are often highly discontinuous while their storage capacity remains a relatively continuous function (as limited by depositional and diagenetic porosity history).

Since 1993, quite a bit of study has gone into identifying the extent and quality of fracture networks and the impact these systems have had on reservoir management, fluid re-injection, and completion efficiency. In West Texas alone, well over 100 short radius horizontal wells have been drilled in one field since 1986. Horizontals drilled in this fractured carbonate were initially done to maximize oil production while limiting gas coning. With the recent fracture studies, emphasis has moved to using horizontal boreholes to connect large flow features from existing wellbores not currently connected. These more recent wells have targeted fracture zones interpreted from flexure maps which are developed from a second derivative analysis of structural surface maps. This paper examines several of the horizontal wells drilled with the intent of cutting the interpreted fracture zones to provide connection to the large flow features of the carbonate reservoir.

Targeting horizontal wells involves a discussion of massive carbonate features as well as discontinuous features. This paper will also discuss how mapping was used to determine flow feature locations. The horizontal drilling techniques used to intersect these targeted flow features will be outlined along with the assessment of flow features while drilling. A discussion of the refinement of the interpretation and the drilling operations involved will follow.

Massive Carbonate Flow Features

What is a massive carbonate?

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