Abstract

Underbalanced (UB) drilling techniques are no longer being used simply to minimize reservoir impairment and improve access to remaining reserves in depleting fields. Traditionally archived and forgotten after the well was drilled, production data and reservoir information gathered while drilling underbalanced are increasingly gaining importance with oil and gas companies.

One Operator has been using UB technology in both clastic and carbonate reservoirs initially to maximize production through the minimization of reservoir impairment.Most of the operator's fields are supported by a weak aquifer or are under tertiary development through waterflood, and the task of water management is a growing concern.While minimizing reservoir damage is now a foregone conclusion when discussing UB operations, the ability to further maximize production through the ability to shut off water-producing zones is now a prime driver for drilling UB.

The comparison of ‘snapshot’ fluid production data during drilling offers a unique perspective on the well being drilled against geophysical LWD/MWD and post-drilling logging data.Temporal production data offers another piece to the reservoir makeup puzzle, and when placed alongside other available data allows reservoir engineers and geophysicists to forensically characterize their reservoir.

Introduction

A UB engineer has always been able to expertly run a multiphase flow simulator and understand all of its vagaries and nuances.Like all things progressive, the multiphase flow simulator has evolved into a very trusted source of interpreted information for drilling UB wells.

The engineer and flow software were especially critical components of a UB operation in the early days of UB drilling.These components were relied upon to determine the resultant bottomhole circulating pressure (BHCP) based on a calculated balance of gas and liquid injection rates, reservoir productivity and surface backpressures in the absence of active wellbore pressure data.

The operator's site representative relied on the size of the flare (or increase in the pit level) and the engineer's calculations to determine not only if the well was underbalanced throughout the wellbore, but by how much. In some cases, wells were drilled in fields where subsurface knowledge was poor - it was only through ‘production steering’ while UB that the sweetspots were found.When all of these factors yielded a positive result, a UB well was deemed successful and the engineer's job complete.

The Evolution of UBD - Forensic Reservoir Characterisation

An Operator's well and reservoir engineers traditionally have relied on inferences made from LWD resistivity measurements to determine the target carbonate location and guide the final well path.Initially production data gathered while drilling UB was used only to quantify its benefits through the calculation of increased productivity index (PI) and tabulation of early oil. The onset of water and the ability to meter and correlate it to a depth became increasingly powerful albeit unexpected pieces of evidence when compared against resistivity.

A composite graph of real-time resistivity variances (red curve) across the open hole of a wellbore from the heel to the target depth is shown in Fig. 1. Deviation from the horizon is shown in yellow.A trend may be seen in the increase in resistivity after drilling out the shoe to an average value almost double the original occurring at about 2700 m.

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