Behind-casing measurements delivered complete formation evaluation on Pemex's Cafeto Sur-1 well, Chilapilla-Jose Colomo asset, Mexico. When conditions prevented openhole logging, and traditional cased-hole logs proved inadequate to clearly identify gas-bearing sands, Analysis Behind Casing (ABC) logs provided the petrophysical, saturation and rock mineralogy information needed to evaluate and assess several promising intervals for future completion and production.

This article discusses the well and the problems it encountered with openhole logging and traditional cased-hole logging interpretation. ABC logging techniques and interpretation solved the problems and salvaged future production potential of the Cafeto Sur-1 well for Pemex.

The Cafeto Sur-1 well

Pemex's Chilapilla-Jose Colomo asset is located in the mature Macuspana basin, southeast Mexico (Figure 1), where most of the fields produce oil and gas from Tertiary sands of the Pliocene and Upper Miocene. Since the early 1900s, several important fields have been discovered in this area. Along with the Chilapilla and Jose Colomo fields, they include the Fortuna Nacional, Usumacinta and Vernet fields.
In 2002, Pemex conducted an integrated study of its Vernet field and determined that prospective horizons existed based on 3-D seismic amplitude versus offset (AVO) and inversion analysis. The study team made recommendations that included drilling an exploratory well, Cafeto Sur-1. In March 2004, Pemex targeted the Vernet-33, three sands situated at about 6,560 ft (2,000 m) vertical depth in a relatively close gas well. The vertical well missed the targeted objectives, so at a depth of about 1,770 ft (540 m) a window was cut in the casing to drill a sidetrack. However, the drillstring got stuck at about 3,940 ft (1,200 m). After several days, the string was freed. Drilling resumed and continued until reaching a depth of about 5,250 ft (1,600 m), where it was planned to set intermediate casing.

Before running the intermediate casing, the acquisition of openhole log data was planned. Although several attempts were made, well control issues prevented openhole logging across the primary reservoir. This and the prior drillstring sticking experiences drove the decision to secure the well by running 7-in. casing, with the plans to acquire the required data via cased-hole logging. Petrophysical and saturation data were needed to identify the sandstone intervals to perforate, test and book new reserves in this exploratory well. Historically, quality formation evaluation measurements have had to be obtained exclusively in an openhole environment.

However, advanced technology has since made it possible to acquire this data in cased-hole wells. During the drilling of a well, if hole stability or well control problems are encountered, operators may decide to set casing soon after drilling if it is too risky to proceed with other openhole operations such as logging. This was the case for the Pemex Cafeto Sur-1 well.

After setting the intermediate casing, it was initially decided to simply run a gamma ray log (GR) and compensated neutron log (CNL). However, Pemex soon realized that CNL-GR logs were not enough. The CNL traditionally has been used as a porosity indicator in cased wells. Although it does provide a good estimate of formation porosity in most conditions, knowledge regarding the rock's lithology and the type of fluids contained in the pore space is also needed to perform an adequate interpretation. For the Cafeto Sur-1 well specifically, the neutron porosity decreased in several intervals, which could have been a qualitative indication of sand (Figure 2). However, it also could have been caused by numerous other effects such as rock compaction. While the mud log signaled several hydrocarbon shows, they were not precise enough to locate the specific intervals to perforate. In addition to the uncertain CNL interpretation and imprecise mud log indication, the GR log was nearly flat because of the nature of the rock mineralogy. Naturally occurring radioactive feldspars in the sand made it almost impossible to locate the productive sands based on this information alone (Figure 2).

ABC logging

Consequently, the traditional cased-hole CNL-GR logs in themselves were insufficient to clearly identify gas-bearing sand, which drove the need to supplement them with additional data. Thus, the Cased Hole Formation Resistivity-Slim (CHFR-Slim) tool was run with other ABC tools, namely the Elemental Capture Spectroscopy (ECS) and Cased Hole Formation Density (CHFD) tools. The first two additional cased-hole logs acquired were the CHFR-Slim and ECS logs. The CHFR-Slim tool provides deep-reading formation resistivity measurements through steel casing in slim holes for either new wells where no openhole resistivity data exists or older wells where bypassed hydrocarbons or movement of fluids need to be evaluated. Its enhanced hardware and measurement principles improve the operational efficiency of cased-hole resistivity measurements and does so with a depth of investigation (DOI) comparable to openhole laterolog devices. This is an important advantage compared to other tools used to compute water saturation through casing, like the pulsed neutron nuclear logging tools typically used for behind-casing evaluation. The CHFR-Slim tool provides resistivity measurements that have a DOI of between 7 ft and 32 ft (2 m to 10 m), which is more than an order of magnitude deeper than measurements produced by behind-casing pulsed neutron tools having shallower DOIs. This is even more relevant in contingency logging cases for new wells like the Cafeto Sur-1, where drilling-related issues can dramatically affect the borehole geometry. With the absence of an openhole caliper log, borehole diameter changes are unknown. As such, a deep reading measurement is needed to confidently see beyond large borehole washouts.

The lithology knowledge required for the CNL interpretation was provided by the ECS tool, which delivers quantitative lithology information for rock mineral contents based on neutron-induced capture gamma-ray spectroscopy. The primary elements that the ECS tool measures are silicon, calcium, iron, sulfur and, by computation, aluminum. Concentrations of silicon, calcium and iron are converted into three main components: total clay, total carbonate and quartz-feldspar-mica (QFM). Specifically, the measured silicon, calcium and iron are fed into a general algorithm to predict clay volume. The calcium concentration log is used to detect total carbonate presence with an accuracy that is not available from any other logging device. Calcium and sulfur logs are used to determine anhydrite. In general, the remaining content of a formation is made up of QFM minerals. If needed, other algorithms can be employed to compute and quantify still other minerals, namely pyrite, siderite, coal and salt.
After running separately the additional CHFR-Slim and ECS logs, the picture became much clearer (Figure 3). The ECS log showed several clean sand intervals, while the CHFR-Slim tool provided a resistivity curve that indicated the sections having hydrocarbon potential. However, doubts remained whether the high resistivity and low neutron-porosity intervals were tight or gas-bearing, as seen, for example, at depths X1,578 ft to X1,584 ft (X481 m to X483 m). Therefore, when the last section of the well was drilled to total depth (TD), the CHFD log was run while pulling out of the hole over the same interval where the CNL-GR, CHFR-Slim and ECS logs had been run previously. With the use of a chemical gamma ray source, the CHFD tool makes accurate formation density measurements in cased wells for different casing and borehole sizes. Its three-detector system makes a density measurement that is corrected for casing and cement thickness.

All together, these combined cased-hole logs showed a more complete picture of the Cafeto Sur-1 well, making it possible to perform a comprehensive evaluation of the rock properties and fluids contained in the porous space (Figure 4). For the section illustrated, there are three main sand intervals. The first one, from X1,345 ft to X1,384 ft (X410 m to X422 m), shows an increase in resistivity but no density-neutron crossover, most probably indicating liquid hydrocarbons. The second interval, from X1,539 ft to X1,568 ft (X469 m to X478 m), has resistivity and density-neutron crossover, indicative of gas. Starting at X1,575 ft (X480 m) downward to the bottom of the section displayed, the last interval, from X1,575 ft to about X1,591 ft (X480 m to X485 m), shows again high resistivity and density-neutron crossover, indicating more gas. Below, the resistivity decreases gradually, indicating a possible water contact. There are some other smaller resistivity peaks and minor density-neutron crossovers between X1,385 ft and X1,539 ft (X422 m and X469 m), possibly identifying other potential intervals to test.

At the time of publication, the logged section illustrated and discussed in this article had not yet been tested because there were several other deeper intervals currently producing considerable amounts of gas, condensate and light oil, opening new possibilities for development. The well's current production will continue until the lower intervals are depleted, at which point the new zones identified will be opened, tested and put on production. Despite the fact that test data has yet to confirm the ABC evaluation, all the behind-casing measurements consistently indicate hydrocarbon-bearing intervals. The combination of resistivity, lithology, porosity and density information enabled clear identification of hydrocarbon-bearing sands. As such, the confidence remains high that favorable production test results will follow.

This field application proved the usefulness of several behind-casing measurements to provide a complete formation evaluation in situations where it is too risky or even impossible to log a well in open hole. Comparing the first set of conventional logs (Figure 3) to the complete ABC dataset (Figure 4) clearly demonstrates that the latter comprehensive cased-hole information provides for much better decision-making regarding well completion and field development for the Cafeto Sur field and fields like it throughout Mexico and around the world.

Future outlook

For the Cafeto Sur-1 application, it can be recommended to also run a wireline-conveyed cased-hole formation tester. The Cased Hole Dynamics Tester (CHDT) provides a technique for determining formation pressures in new or older cased wells, and it enables efficient, cost-effective fluid sampling without the inherent risks of standard sampling techniques. Its fluid sampling capability would have verified reservoir fluids, while its ability to determine pressure gradients would have identified contacts.

The innovative cased-hole formation tester tool seals against the casing and uses a flexible drill shaft to penetrate through the casing and cement and into the formation. Its use eliminates the need for explosives altogether. Downhole sensors measure formation pressure, pressure transients and formation fluid resistivity. Combining the cased-hole formation tester tool with various modules of the Modular Formation Dynamics Tester (MDT) enables high-quality sampling, enhanced fluid identification and contamination monitoring. After all measurements and samples have been taken, the tool inserts a corrosion-resistant metal plug into the hole that had been drilled in the casing, thereby preserving casing integrity and eliminating the need for costly repair procedures.

Cased-hole logging as well as cased-hole formation pressure testing and fluid sampling will continue to offer reliable, representative reservoir information that historically could only be performed in the openhole environment.

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