Gas-Cap Monitoring in High-Flow-Rate, Low-Porosity Clastic Reservoirs in Colombia Gives Big Economic Returns
- David Bullion (BP Exploration Colombia) | Fabio Gonzalez (BP Exploration Colombia) | Marvin E. Markley (Schlumberger)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Evaluation & Engineering
- Publication Date
- June 2001
- Document Type
- Journal Paper
- 240 - 245
- 2001. Society of Petroleum Engineers
- 3.3 Well & Reservoir Surveillance and Monitoring, 5.6.1 Open hole/cased hole log analysis, 3.3.1 Production Logging, 3.2.2 Downhole intervention and remediation (including wireline and coiled tubing), 5.1.1 Exploration, Development, Structural Geology, 5.2.1 Phase Behavior and PVT Measurements, 2.4.3 Sand/Solids Control, 4.6 Natural Gas, 1.6 Drilling Operations, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.6.4 Drillstem/Well Testing, 5.4.2 Gas Injection Methods, 2.2.2 Perforating
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Cased-hole neutron logging has been in use for many years and is often used to monitor gas movement in reservoirs behind pipe. This works well in high-porosity reservoirs but is rarely successful in low-porosity reservoirs. In Colombia, there are high-flow-rate reservoirs with significant permeability but low porosity [5 to 6 porosity units (p.u.)].
The challenge was to monitor gas-cap expansion, fluid movement owing to re-injection, and voidage. At the same time, the objective was to identify low gas/oil ratio (GOR) intervals behind pipe for possible recompletion.
A number of different examples illustrate both the technique and the economic benefit of the monitoring program. Many of these wells contributed significant additional oil from bypassed zones that were initially thought to be dry gas. In some wells, the instantaneous incremental production was over 10,000 BOPD from a single recompletion.
Gas-cap monitoring in very-low-porosity reservoirs in Colombia proved to be not only feasible, but also extremely profitable by differentiating dry-gas zones from low-GOR zones.
Introduction - Cusiana Field, Colombia
Cusiana is a field in Colombia (Fig. 1) that produces high rates of oil and gas from sandstone formations in the Andes foothills. Tectonic evolution of the area dates back to an early Paleozoic rifting period, forming grabens where a thick sequence of sediments was deposited. The Llanos (the plains area east of the Andes in Colombia) was an extensional subsidence basin in the Triassic and early Cretaceous periods, becoming a passive subsidence basin in the late Cretaceous period (Fig. 2).
Tertiary, Cretaceous, and Paleozoic silico-clastic sedimentary sequences overlie the pre-Cambric basement consisting of crystalline and metamorphic rocks. The maximum sedimentary thickness reaches 12 000 m.
Exploration in this basin has focused on structural traps in the foothills and platform areas where the major discoveries have been made. The Cusiana volatile oil field is associated with a complex thrust anticline and overturned structures that exist along the foothills zone.
The productive formations in Cusiana (Mirador, Barco, and Guadalupe) have low formation porosities, but they have relatively wide permeability ranges. The Mirador has average reservoir porosities in the 5 to 6 p.u. range, with very short intervals of higher porosities up to 10 p.u. and permeabilities up to 1,000 md. The Barco formation has porosities from 5 to 10 p.u. and permeabilities from 1 to 300 md. The Guadalupe has 6 to 15 p.u. and permeabilities of approximately 200 md. The wells drilled into these formations produce from 5,000 to 25,000 BOPD. The generalized stratigraphic column in Fig. 3 shows the productive formations and their ages. The map in Fig. 4 shows the field structure, which trends northeast/ southwest. Additionally, the well locations surveyed are represented.
Historically, neutron porosity has been computed from a ratio of the near- to far-count rates. Generic neutron tools have a chemical nuclear source that emits high levels of neutrons to two detectors. Using the ratio of the near-to-far counts from these detectors has the advantage of a robust calibration repeatability and accuracy. This method, however, tends to mask the sensitivity of the source detector count-rate measurements to many effects, including gas and certain borehole effects.
It was decided to use a new model neutron tool with four sets of detectors, in an array with a downhole neutron accelerator, as a neutron source to monitor gas movement in Cusiana. This tool is called the Accelerator Porosity Sonde (APS, Fig. 5). It was expected that the increased source-to-detector spacing would give good results in this difficult, low porosity environment. The array of source-to-detector spacings was designed with Monte Carlo simulations to optimize its effectiveness.
The detectors are eccentered (rather than centered in normal neutron tools) to optimize their responses and minimize environmental effects. It was also expected that the relatively higher neutron source strength from the downhole neutron accelerator would give improved sensitivity to gas. Using the individual count rates from the near and far epithermal detectors in a qualitative overlay gives a superb sensitivity to gas in the formation, even at very low formation porosities.
A formation sigma was also available from the APS tool. The formation and injected waters in Cusiana and Buenos Aires are very fresh, and sigma is not an effective water-to-hydrocarbon indicator. The porosity range in the Mirador and Barco formations is too low to use sigma as a reliable gas discriminator. The difference between 100% gas and 100% oil at this porosity level is 0.84 capture units.
Why Does This Work?
The far epithermal detector reads much deeper into the formation than the near epithermal detector because of the longer source-to-detector spacing. The near epithermal detector gives shallow readings and is not much affected by the deeper formation. In gas zones, there are fewer hydrogen atoms to slow down the neutrons, so there are many neutrons that reach the far detector. The near epithermal detector is less affected by this gas. In general, most neutron tools are affected by gas in a similar manner; however, in contrast to the use of more stable ratios for a quantitative porosity, the count-rate overlay approach with the different epithermal detector spacing is more sensitive to gas and, as a result, an excellent indicator of gas in the reservoir - even through casing.
The technique is to overlay near and far epithermal count rates in a liquid-bearing interval, then look for gas when the far detector count rate increases faster than the near-count rate. The approach is even more robust when there are in-gauge shales that can also be used as a low-count rate endpoint. Separation in gas intervals or intervals with gas in the annulus is very pronounced. The tool count-rate results were not compared to other generic neutron tools to determine if a less sophisticated measurement may have worked with only thermal neutron count rates in low-porosity reservoirs. The authors do not know of any other monitoring projects that use the APS.
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