SWAT SPECTRAL WELL ANALYSIS

SPECTRAL GAMMA RAY

Records entire spectrum of all natural emitting gamma rays and previously injected isotopes during stimulation (frac). Each isotope energy level, measured in electron volts (KeV), is simultaneously recorded at the surface.

The resulting spectral provides vital data to determine the placement of all frac fluids and/or proppants.

The spectral survey can be observed on a remote monitor while the downhole spectral survey is being recorded. Does not have to retrieve spectral gamma ray from downhole before getting survey results.

Other services recorded on same trip downhole are correlation gamma ray, temperature survey, and collar log for depth control with other surveys.

Isotopes Commonly Used for Tagging

Isotope                 Half-Life                 Gammy Ray

Energy (KeV)

Iridium-192           74 Days                 317,468,604

Scandium -46       84 Days                889,1120

Antimony -124      60 Days                603,1691

Iodine -131           8 Days                  364

NO WASH ISO SEAL

“NO WASH” PRODUCT

(IsoSeal-3) “NO WASH” is designed to eliminate radioactive residue in fluids or tubular goods in wells while being fractured, acidized, or cemented. The basic material (beads) has a lower density and higher sheer stress tolerance than other types of products being used. This material will not soften or disintegrate when immersed in liquids.

Proprietary processes are used to develop IsoSeal3 “NO WASH” into a superior product by a three-step formula.

I. Absorbent material (beads) are applied with appropriate compounds to make the desired radioactive isotope. (Examples: Iridium-192, Scandium-46, Antimony-124)

II. Sealant is applied by a special process to ensure complete encapsulation.

III. Sealed, encapsulated beads are shipped to a nuclear reactor for radioactive activation.

FEATURES OF ISOSEAL-3 “NO WASH” MATERIAL:

Has a lower density for use in almost all downhole frac or acidizing jobs.
Minimizes flowback of radioactive beads because it is made of solid particles with high shear tolerance.
Has low density, allowing for injection on the down stream side of frac or acid pumping equipment. This eliminates the possibility of a service company’s equipment becoming contaminated.
(IsoSeal-3) “NO WASH” has combined these features to allow longer periods before logging.

THE DETERMINATION OF FRACTURE ORIENTATION USING A DIRECTIONAL GAMMA RAY TOOL

The Determination of Fracture Orientation Using a Directional Gamma Ray Tool, J.L. Taylor, III, et al, SPWLA 91-AA, June 1991.

ABSTRACT

The effectiveness of hydraulic fracturing operations is commonly evaluated by tagging the materials pumped downhole with one or more gamma-ray-emitting isotopes and subsequently logging the borehole with a gamma ray spectroscopy tool. Many times it is very desirable to also determine the orientation of the fracture planes. This paper describes a directional gamma ray tool which makes this possible.

The main tool component is a sodium iodide scintillation detector within a rotating tungsten shield containing a slotted aperture. A three-axis accelerometer is used to determine the direction of the gravity vector relative to the tool axis. The 1-11/16-inch-diameter toolstring consists of a gamma ray spectroscopy tool and a directional gamma ray tool. Optionally, a direction gyro survey tool may be attached.

The logging procedure is first to run the spectroscopy tool to determine the distribution of tracers. This allows depth intervals to be selected for stationary measurements and moving runs with the directional tool. Example logs from prototype tool field test illustrate the effectiveness of the directional gamma ray measurements. These results show that many gamma ray maxima exhibit significant azimuthal asymmetry. The interpretations of these asymmetries are discussed and compared with laboratory measurements.

GAMMA RAY TRACERS HELP EVALUATE ACID DIVERSION

Gamma Ray Tracers Help Evaluate Acid Diversion, J.L. Taylor, III, et al, Petroleum Engineer International, February 1990.

ABSTRACT

The use of multiple gamma ray tracers has helped evaluate acid diversion in several North Sea completions. The use of multiple radioactive tracers and subsequent logging with advanced gamma spectroscopy techniques offers a cost-effective and convenient method for direct measurement of vital stimulation parameters such as diverter effectiveness.

Productive intervals in the Norwegian sector of the North Sea tend to be quite thick: Danian pay ranges up to 550 ft, and Maestrichtian up to 500 ft. Average porosity can reach 48%, and matrix permeability varies from less than 0.1 to 5 md. Well productivity seems dependent on the degree of natural fracturing, and pressure transient testing derived permeability estimates can be as much as 75 times the matrix permeabilities obtained from core measurements.

Perforations are placed in 10-to 20-ft clusters spaced 40 to 80 ft apart, with a shot density of 2 shots/ft throughout each cluster. The acid stimulation treatments are then pumped in multiple stages, with each stage consisting of a viscous pad, acid, overflush, and diverter (ball sealers are most often employed).

The tracer studies outlined in this article were conducted on six stimulation treatments to determine if the diverter techniques employed result in relatively even treatment of all pay, and to evidence the creation of multiple hydraulic fractures. All evidence suggests adequate diversion usually occurs and new fractures are propagated on each stage.

The specific tracer technique used involved the placement of a different discernible gamma emitting tracer in each stimulation stage to determine its relative placement and thus infer the effectiveness of the diverter stages. Three tracers, Antimony (124Sb), Iridium (192Ir), and Scandium (46Sc) were added to each stage to differentiate the placement of up to three stages or groups of stages. Following each treatment, a Prism® log was run to identify tracer placement. A detailed description of the materials used and the tagging and logging techniques were discussed in earlier articles.

The tracers were prepared as ceramic particle encapsulations, with a mean particle size of 0.5 mm. This proprietary preparation exhibits a tracer washoff of less than 0.01% in 28% HC1 at 100xC, and has a specific activity of approximately 0.89 mCi/gm (32.8 MBq/gm) or 0.0014 mCi/particle (0.0527 MBq/particle). The use of these tracers in particulate form was preferred to using soluble forms to minimize environmental concerns of returning radioactive residue to the surface with the flowback of the spent acid. The tracers and the equipment used to inject them into the stimulation process were transported to the well platforms from the UK aboard the service company’s vessel performing the treatment. Generally, about 20 mCi (740 MBQ) of each tracer was injected continuously throughout each acid stage. Specific licensing to perform the radioactive tracer studies was required from Norway’s National Institute of Radiation Hygiene.

The wells were logged using a 1.6875-in. (4.2863-cm) OD Prism tool, which contains a 1-in. by 6-in. scintillation crystal. The logging speed was 500 ft/hour (152.4 m/hour). At each 3-in. (7.62 cm) interval, the entire 256-channel gamma ray spectrum was encoded and transmitted to the surface and recorded on magnetic tape. This data was subsequently processed using the proprietary software on a microcomputer at a log analysis center in Stavanger, Norway. The software mathematically unfolds the gamma ray spectrum to determine tracer yields and indicate the location of individual isotopes along the wellbore. Furthermore, the program determines the lateral tracer placement (inside or outside the casing) by using a photopeak to downscatter ratio.

The results of the six tracer studies are presented in tabular form in Table 1. The Prism logs from wells A, B, and C are presented as Figs. 1,2, and 3, respectively.

In summary the following conclusions are made:

Tracer materials of the type and packaging used are effectively placed in the formation and do not flow back into the well. In consequence, reliable Prism data may be obtained in one pass after cleanup flow of the well.
Where the cement bond log indicates effective mechanical isolation of perforated zones in the treated interval and the number of perforations is low, good diversion occurs.Breakdown of both single and multiple zones on individual stages were observed.
Limited fracture heights and formation of multiple fractures occurs.
Tracer material positioned during the early treatment stages is partially stripped away during the later stages. This is particularly apparent when the number of perforations is low and flow velocities will, in consequence, be high.
The logging technique and analysis allows us to determine the placement of isotopes in the presence of radioactive scale.

TRACERS IMPROVE HYDRAULIC FRACTURING

Tracers Can Improve Hydraulic Fracturing, J.W. Chisholm, Petroleum Engineer International, July 1989.

ABSTRACT

Because the success of well stimulation treatments often dictates the economic justification of petroleum field development, much effort has been devoted to the measurement of various parameters associated with this critical and costly operation. Specifically, the prediction, measurement, and optimization of induced hydraulic fracture geometry is an endeavor which has resulted in a major industry-wide research effort. in the past 10 years, extraordinary advances have been made and the evolution of well stimulation technology is still proceeding at an incredible rate.

Many methods of actually measuring or inferring fracture geometry during or after a frac treatment have been developed and tested; however, few are considered sufficiently pratical, convenient, and cost-effective to be performed routinely. Analysis of pressure data from frac treatments and prefrac injection tests can lead to quantification of certain fracture parameters such as closure stress, fluid efficiency, and leakoff coefficient; however, computation of most of these properties requires knowledge of the vertical fracture height.

Of all the available vertical fracture height measurement techniques, post-treatment tracer and temperature surveys are by far the most common because they are convenient and relatively inexpensive to conduct. Temperature surveys can provide quantitative vertical fracture height determinations; however, they are plagued by the following problems:

Cross flow and pressure-induced fluid redistribution following the treatment can result in temperature surveys that are difficult to interpret.
In wells where the formation temperature differs only slightly from the surface ambient temperature, these surveys are not possible.
If significant amounts of proppant remain in the wellbore and must be circulated out before logging, the circulation process may distort the temperature anomalies created by the frac treatment, or the temperature anomalies created by the treatment may completely dissipate by the time the temperature survey can be conducted.
Because of these problems, particularly the last, frac treatments are frequently tagged with radioactive tracers. The major objections to using gamma emitting tracers have been that:

Only single tracer operations were pratical, unless tedious multiple logging runs using tracers with greatly differing half-lives were conducted.
A conventional gamma ray log cannot differentiate tracer material actually placed in the formation from residual tracer left in the wellbore; thus, the determination of actual vertical fracture height is often obscured.
The depth of detection from the wellbore is limited to less than a meter unless excessive concentrations of radioactive tracer are employed.

USING TRACERS TO EVALUATE PROPPED FRACTURE WIDTH

Using Tracers to Evaluate Propped Fracture Width, S.A. Holditch, David Holcomb, Zillur Rahim, SPE 26922, November 1993.

ABSTRACT

Using Tracers to Evaluate Propped Fracture Width

Many production engineers are beginning to use three-dimensional (3-D) fracture propagation models to design and analyze hydraulic fracture treatments. To use a 3-D model, one must define the layers that comprise the reservoir and develop detailed datasets that accurately describe the layers. The data that are critical for designing and analyzing hydraulic fracture treatments are in-situ stress, formation permeability, formation porosity, reservoir pressure, and Young’s modulus. Many times, these parameters can be determined from logs and/or correlated to lithology.

Once the datasets are obtained, one can use a three-dimensional fracture propagation model to estimate values of created or propped fracture length, width, and height. To understand and improve the fracture design process, the engineer must confirm the estimates of fracture dimensions that are predicted by a fracture propagation model. To verify the model, one must analyze field data to be sure the field data are consistent with the model results. For example, the net pressure predicted by the 3-D fracture propagation model should closely match the net pressures observed in the field. When net pressure is adequately matched, we usually find that the overall created fracture dimensions predicted by a 3-D fracture propagation model are reasonable. To determine estimates of propped fracture length, one must also analyze post-fracture production and pressure transient data. Because of fracture fluid cleanup problems, we often find that values of propped fracture length generated by analyzing field production data are much shorter than the created fracture length predicted by the fracture propagation model. Detailed engineering studies are often required to reconcile the differences.

To directly measure values of fracture width, one must perform a fracture treatment in openhole, then use a downhole imaging tool to “see” the fracture. Such an approach is not usually practical. In this paper, we will describe a method to qualitatively estimate the propped width profile at the borehole that uses radioactive tracers. Confirming the propped width profile generated by a model with field data can be very beneficial and informative.

We have found that the use of zero wash radioactive tracers can help us learn both (1) where the fracture fluid is going and (2) where the proppant resides in the fracture near the wellbore. Assuming the level of radioactivity is proportional to volume, then the level of radioactivity will also be proportional to the propped fracture width. As such, one can obtain qualitative estimates of propped fracture width at the wellbore using a radioactive tracer where the strength of the radioactive signal is proportional to fracture volume near the wellbore.

The objectives of this paper are to discuss what factors control the fracture width profile and how to obtain data to compute fracture width. We also explain how one can use radioactive tracers to develop data that can be analyzed to determine qualitative estimates of propped fracture width. Finally, we provide several examples to illustrate how one can use estimated values to calibrate a 3-Dimensional fracture propagation model.

The information described in this paper can be used by a production engineer to obtain a better understanding of a specific hydraulic fracture treatment. As our understanding of hydraulic fracturing improves, we should be able to design the optimal fracture treatment with more certainty. When we design and pump the optimal fracture treatment, we maximize the economic return on developing oil and gas properties.

TRACERS FACILITATE STIMULATION JOB EVALUATION

ABSTRACT

Radioactive Tracers Facilitate Stimulation Job Evaluation

Logging tools can now quantify multiple isotopes, including the volume of individual isotopes present and their radial position away from the wellbore. In conjunction with those improvements, tracers have been developed that eliminate “wash off” effects of conventional tracers. By precisely locating the presence and concentration of traced proppant at the wellbore, better evaluations can be made of vertical and radial proppant distribution near the wellbore and fracture aperture width.

A comprehensive study of 98 wells with 136 fracture stages in four different basins has been completed. Each stage was traced and logged. Spectral gamma ray logs were compared with conventional openhole logs, sonic stress logs where available, and cased hole logs such as cement bond and production logs. This data was then compared on a well-by-well basis with the fracture design program, post treatment stimulation reports, and production history.

Several trends were identified while building this massive stimulation evaluation database. Problems that potentially could be solved using tracer technology are:

• Fracture height greater than design
• Unstimulated perforation sets within a stage
• Understimulated pay intervals

USING TRACERS FOR MONITORING AND DIAGNOSING HORIZONTAL WELL STIMULATIONS

ABSTRACT

Using Tracers for Monitoring and Diagnosing Horizontal Well Stimulations, David Holcomb, Robert A. Woodroof, World Oil Horizontal Well Completions Symposium, 1996.

The application of multiple radioactive tracers (Zero Wash®) and spectral gamma ray imaging has allowed for improved diagnostics of stimulation treatment distribution. Whether acidizing and diverting or fracturing and proppant placement, multiple tracers (i.e.; Iridium-192, Scandium-46, Antimony-124) have allowed operators to better analyze proppant entry with respect to stage, volume, and/or type placed across lateral intervals, as well as acid entry and distribution in order to better understand and optimize treatment techniques such as diverting, rates, stage sizes, etc.

Holcomb and Read demonstrated that tracers were useful in evaluating Austin Chalk and Bakken Shale completions in South Texas and North Dakota respectively. Qualitative comparisons helped operators understand stimulation coverage and diversion effectiveness.

Problems still plague the use of tracers in horizontal wells and usually center around uncemented or poorly cemented casing. Tracer materials can accumulate behind pipe in depressions or washed out sections even if acid or slickwater treatments are overflushed. While this may make tracer images more difficult to interpret, it does not rule out their usefulness for identifying potential problem areas. Open hole horizontal completions have also posed problems for tracers due to wash-off of tracer materials and adsorption onto rock, not necessarily associated with fracture entry. Improvements made in horizontal well drilling and completions have been aided by the reliability of improved Zero Wash® tracer carriers and spectral imaging tools to provide a more quantitative look at stimulation treatment placement across horizontal well sections without the problems associated with wash-off and subsequent adsorption onto rock, casin, liners, etc.

One particular application has been noted with tracers used to confirm the success or failure of various diverting techniques to allow lateral zones to be completely acidized. Different Zero Wash® tracers are placed in different stages of acid separated by various diverter stages using such materials as oil soluble resins, gel pills, ball sealers, benzoic acid, rock salt, crushed Unibeads®, or foams. Three tracers are usually used in a variety of carrier sizes, densities, and non-wash/crush/abrasion-loss formats. They include Iridium-192, Scandium-46, and Antimony-124, with half-lives varying from sixty to eighty-four days.