No abstract

The deep reservoir sandstones of the El Furrial and Pirital oilfields, located in northern Monagas state, eastern Venezuela, are famous for their high degree of geological complexity and heterogeneity. Typical well completions in this area consist of natural flow from multiple intervals through perforations in cemented casing. However, conventional perforating guns used in these completions cause additional near-wellbore formation damage that adversely affects well productivity and results in reduced productivity indexes in these wells.There have been significant efforts in recent years to implement new methods and technologies to produce cleaner perforations and better production rates. One of these technologies is a new dynamic underbalance perforation system which allows precise management of the underbalance differential pressures to produce a relatively clean perforated tunnel while minimizing formation damage. Application of this system results in improved connectivity between the reservoir and wellbore which allows substantial improvement in oil production rates.Formation petrophysical properties are used to determine whether an interval is a suitable candidate for application of this technology. A separate nodal analysis model is created for every reservoir interval in each well. These models are designed to represent the actual and expected production conditions and account for reservoir heterogeneity. In the El Furrial and Pirital fields, reservoir properties can vary widely: permeabilities from 0.1 to 1,000 mD; porosities from 9 to 18%; oil gravities from 22º to 32º API; static pressures from 6,000 to 8,000 psi; and confined compressive strengths from 7,000 to 20,000 psi. This paper presents case histories from wells in eastern Venezuela to illustrate the production benefits derived from this advanced perforating system technology. In three wells, supporting evidence demonstrates that near-wellbore damage caused by this new innovative underbalance perforation system is close to zero, resulting in over 100 % increase in the productivity index. In the fourth case history, we discuss the reasons why the actual well production rates did not achieve the rates projected by nodal analysis. IntroductionDiscovered in 1986, the El Furrial field is currently considered to be one of the largest oil producing fields in Venezuela, with a current production rate of approximately 425,000 BOPD. Located on the north flank of the eastern Venezuela basin (Fig. 1), the field is currently produced and operated by Petroleos De Venezuela S.A. (PDVSA). Since its discovery, it has produced over 2 billion bbl of approximately 25 o API oil. Proven reserves total over 4.1 billion bbl and estimated oil in place is more than 8.6 billion bbl.

This paper compares production results and the evaluation of conventional logs versus advanced logs as nuclear magnetic resonance (NMR) in a mature field, discusses vertical reservoir heterogeneity and different flow units in apparent homogeneous reservoirs, and identifies bypass reserves oil resulting from poor sweep efficiency. A petrophysical evaluation was performed using conventional and advanced logs. Comparing the results with dynamic production logs demonstrated that the NMR evaluation was more accurate than conventional log evaluation in identifying different flow units. Traditional interpretations with conventional tools did not identify the water entry problems caused by different flow unit systems, which resulted in production sweep effects from nearby wells. NMR identified this problem, the reservoir quality, and the vertical heterogeneity and its effect on water entry. It was determined that a uniform sweep did not exist from the bottom to the top of the reservoir; thus, the aquifer water entry depended exclusively on reservoir quality related to flow unit characterization and capacity, as permeability and seal distributions are locally developed. Adding to this complex scenario of identifying bypassed zones and the noncorrelation with conventional logs is carbon dioxide (CO2) hiding the presence of the true porous zones, potentially leaving remaining reserves that could be overestimated or underestimated. Comparing dynamic logs versus static logs resulted in a credible endorsement of NMR, demonstrating its advantages and accuracy over conventional logs and a better technical application for recovering the remaining reserves and optimization of a mature field.

Although leakages in well tubulars have always existed, their occurrence has become very frequent as the number of active wells in mature fields increases. The catastrophic risk of these leaks is an increase in the number of environmental accidents in the oil and gas industry. One of the fundamental causes of leaks is corrosion, which plays a negative role in the productive life of the wells. Generally, these environmental events are associated with surface or near-surface sources. Since multiple casing strings exist within this depth range, the identification of the leak location becomes extremely difficult. In view of this, the industry has put much effort in improving and new technology to be more precise and comprehensive in diagnosing these leaks. The evolution of two of such technologies will be addressed in this paper. The first one is a new electromagnetic high-definition frequency tool for pipes and multiples casing for metal loss detection. This state-of-the-art technology is a noticeable improvement over existing tools, due to an important increase in the number of sources, number of detectors and wide range of working frequencies. The combination of these changes allows for the evaluation of metal loss in up to 5 concentric casings in a single run. Furthermore, the tool is small in diameter which makes it compatible with production pipes without the need of a workover rig. This versatility obviously helps in the preworkover diagnosis before deciding to move a rig to location to eventually remedy any leak problems. The electromagnetic technology is complemented, with the latest leak detection acoustic technology. A spontaneous audio source is normally associated with downhole fluid movements. The tool has an array of 8 hydrophones with a working frequency range from 100 Hz to 100 KHz. These two different technologies based on independent fundamental principles, allows for the detection of leaks in multiple concentric pipes with great vertical and radial precision to identify the exact location of leaks as small as to 0.02 L/min. the depth of investigation of the system is up to 10 feet. Therefore, it is possible to detect fluid movement within the formation. Pulsed neutron technology was included in the study to detect water movement behind the casing to establish the flow path to the surface in addition to the leak point. A very complex acquisition program was established that was undoubtedly a key success factor in the results obtained. The electromagnetic tool determined the depth of severe casing metal loss in 7-inch casing, also the acoustic tool detected the noise of fluid movement in the 7-inch annulus, and the pulsed-neutron tool showed the beginning of water movement at the same interval the temperature log, also included in the same tool string showed a considerable change that correlated with all these logs, indicating the point of communication in this well. After establishing the uniqueness of the solution, this diagnosis helped the operator define an intervention plan for this well, and to make the appropriate corrections in the field development strategy.