Relative Injectivity Profiles of Irregular Size Holes
Authors: Gore George L. 
Published: December 2007

Relative injectivity surveys of secondary recovery-input wells aid in evaluating the efficiency of a waterflood operation, and indicate what proportion of the injected water may be escaping into unproductive zones. Conducting such surveys in an irregular size openhole presents difficulties, but an effective technique employing radioactive tracers has been developed to meet this problem. Tubing is installed in the well and radioactively "tagged" brine is injected down the annulus. The progress of the radioactive interface down-hole is followed by a gamma-ray detector tool, lowered down the tubing on wire line. When all of the radioactive brine has been displaced into the formation, injection down the tubing is commenced, maintaining the same injection rate. Fresh radioactive tracer is injected into the flow stream and the resulting radioactive interface is again followed through the annulus by the gamma-ray detector, upwards from the bottom of the hole. The elapsed times required for the interface to travel between successive depth intervals, in each direction, are used to calculate the injection capacity of each interval, and the effective diameter of the hole at the corresponding depth.

Introduction

The relative injectivity profile is the petroleum engineer's most valuable tool in evaluating the effectiveness of a particular secondary recovery input well. It enables him to observe the distribution of water intake over the various exposed zones in the well. It enables him to determine if the injected water is truly entering the pay formation to be flooded, or whether it is escaping into undesirable zones. It will reveal differences in injectivity resulting from formation fractures, localized formation damage, variations in permeability, and other formation heterogeneities. An injectivity profile can reveal faulty cement, accompanied by channeling behind the casing. Many of these conditions require remedial work-over treatment, and the success or failure of such operations often depends on the accurate pin- pointing of the difficulty, so that the appropriate remedial measures can be taken.Many techniques have been devised for conducting subsurface surveys, to determine the injectivity profile of a well. Some of these are based on measuring comparative flow rates at different depths, using various types of subsurface flow meters (i.e., temperature, spinner, hot-wire, etc.). Other methods are based on the detection of the interface, moving or static, between two dissimilar fluids being pumped simultaneously down the well tubing and casing (i.e., brine-fresh water, clear-opaque water, radioactive-nonradioactive water, etc:). When such surveys are conducted in perforated casing or relatively uniform openhole, the surveying techniques are relatively simple and uncomplicated. However, when the wellbore is highly irregular, due to nitroglycerin shooting, extensive fracturing, or massive sloughing during drilling, specialized techniques whose accuracy will not be affected must be employed.Some of the better known techniques used for this purpose include the expansible-packer flowmeters developed by Humble Oil and Refining Co. and Socony Mobil Oil Co., the "isoflow survey" developed by Texaco, Inc., and the brine-fresh water profiling technique developed by Sinclair Research Laboratories, Inc.The Humble and Socony Mobil methods employ a down-hole flow-meter, which can be lowered through tubing, and contains a packer which may be expanded to provide a seal in wellbores up to 9 in. in diameter, diverting the entire flow through the flow-measuring device in the tool.The Texaco method consists of injecting brine down tubing and annulus simultaneously. One of the two fluids contains a radioactive chemical. A ratio is established between the two flow streams, and the resultant static interface is detected by means of a gamma-ray tool, lowered through the tubing. This procedure is repeated using different injection ratios, and the injectivity profile is determined from the resultant data.The Sinclair static interface method is similar, but utilizes a brine-fresh water interface, rather than a radioactive tracer. The location of the static interface, at each injection ratio, is determined by means of a resistivity-measuring element attached to the bottom of the tubing string, which is raised or lowered from the surface by means of a pulling unit. Measurements are recorded from the tubing tally.A different technique, utilizing the same equipment, consists of pumping the entire volume of injected fluid down the annulus, determining the rate of fall of the interface at successive depths until the interface ceases to move.

JPT

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