The objective of the paper is to present field results obtained through the evaluation of the Micro-bubble Aphron system, as the drilling fluid in wells VLA-1321, VLA-1325, VLA-1326, VLA-1327, VLA-1329, VLA-1331, VLA-1332, VLA-1334 and VLA-1335 (wells corresponding to Lagomar Integrated Laboratory). This fluid does not require equipment such as compressors, rotary wellhead, equipment to generate and/or inject nitrogen and permits the taking of electric logs conventionally. During drilling, mud weight varied between 6.8 to 7.9 ppg and very low-pressure zones were run through, with equivalent gradients oscillating between 2.4 ppg and 6.4 ppg, without any problems of loss of circulation. The micro-bubble fluid presented excellent inhibition values to clay and shale since the Miocene sands of Basal La Rosa and the alternating sand and shale of the Eocene reservoirs were drilled without any problems. Moreover, in well VLA-1321, this lithology was with an exposure time of 25 days, without any instability problem. On the other hand, in wells VLA-1329, VLA-1331, VLA-1332, VLA-1334 and VLA-1335 all the intervals were drilled from 1520' to 6900', without using intermediate casing and without great problems, which cut down drilling time to 31.0 days with a saving of 4.7 MMUS$ for the corporation. It should be pointed out that the fluid has presented rheological properties, which have permitted drilling with excellent hole cleaning. During the drilling of well VLA-1321, 90' of samples were taken in the Miocene and 300' in the Eocene sands with a high percentage of recovery (90.80%) and in well VLA-1326, 411' of samples were cut with a percentage of recovery of 90.4%. Additionally, the electric logs were not affected by the micro-buble mud and caliper logs showed a hole of excellent gage. Judging from the excellent results obtained, the Micro-bubble Aphron system is still being evaluated in the Eocene mature reservoirs of Lake Maracaibo. Introduction The Lagomar Integrated Laboratory (L.I.L.) comprises a first stage with the drilling of five vertical wells in order to increase the recovery up to 10% of the POES in the next five years, in the La Rosa Basal sands and "C" of the Eocene through the application of forefront technologies that permit high fluid volumes in the mature reservoirs and with low pressures. Due to the low gradients of the Eocene sands, the use of a drilling fluid which guarantees the integrity of the hole, without the risk of loss of circulation and with a minimum damage to the productive formation, is necessary. Recently, experiences have been had with aerated fluids, but the wells presented serious operational problems, such as: lost circulation, caving in of the holes due to the presence of unstable shale, impossibility of running logs, pressure taking, taking of samples, etc. A new system has been developed, a water base drilling fluid called Micro-bubble – Aphron, designed to drill low pressure mature reservoirs; this system is characterized as having in its continuous phase, high viscosity at low shear rate and containing, as internal phase, micro air or gas bubbles, non coalescing and recirculatable. These Micro-bubbles denominated "APHRONS" are generated by the use of a chemical surfactant that traps the air present in the system (active tank) and/or that is generated at pressure drop created by the jets of the bits. This fluid does not require an external source of injection of air and/or gas (compressors, equipment to generate and/or inject gas, etc.). The Aphrons permits reducing the density of the continuous phase to lesser values than water and the use of balance drilling technique.
This paper shows the results of a successful application of the addition of hollow glass spheres, also known as glass bubbles, as a density reducing agent in a drilling fluid. In this field application, glass bubbles were used in combination with an oil based drilling fluid (Core-Drill-N). It was corroborated that the fluid-glass bubble mix is stable, homogeneous, and is compatible with conventional mud motors, bits and surface cleaning equipment.. The system has good rheological and filtration control properties and is suitable for drilling low pressure reservoirs, low permeability and pay zones. During this field application in the well MOT-25B in Venezuela, the density of the base fluid was maintained between 7.1–7.3 ppg (near balance condition) with calcium carbonate as bridging agent. This technology is an alternative to the use of aerated fluids where the reservoir requires a fluid density between 6.0–7.5 ppg, offering some economical and technical advantages due to the elimination of surface compressing and air injection facilities, and to the simplification of operations required to avoid excessive overbalance during drillpipe trips. Additional potential benefits of this low-density fluid include torque reduction as a result of higher lubricity, higher penetration rates and decreased formation damage due to lower invasion of drilling fluid. Glass bubbles are also an alternative to decrease the density of water based drilling fluids, polymer-based fluids, emulsion systems and brines. Laboratory tests were also carried out with different concentrations of glass bubbles in order to evaluate potential field substitutes for aerated fluids in wells which might require lower density fluids. Several formulations for the systems mentioned above were developed with the purpose of achieving maximum density reduction without affecting some mud properties. Fluid density as low as 6.0 ppg was obtained from a 100% oil base mud. Introduction In the last few years in Venezuela there has been an increase in the drilling activities in low-pressures reservoirs with low-permeability, where the utilization of drilling fluids with a density higher to the required could result in a partial or complete loss of fluid into the formation. An excessive level of overbalance could cause effects such as an increase in drilling costs, potential fracturing of the formation, formation damage, and finally potential well loss. The drilling of the above mentioned depleted reservoirs require the use of lower density fluids with specific gravity less than 1 (8.33 ppg). These fluids, in principle would allow maximum extraction while minimizing damage to the producing formation. This work shows the results of the application of glass bubbles as a density reducing agent in an oil base drilling fluid under near balance conditions in an inclined well, drilled in Motatán field in Western Venezuela. Fig. 1 shows Motatán field, in Western of Venezuela. Motatán field is characterized by reactive shale, depleted formations and highly fractured reservoirs with high axial stresses, known for deferred oil production due to flushing by lost circulation. This reservoir has a typical range for matrix permeability from 50 to 100 mD. Porosity in this area is 14% approximately. In addition, this paper presents laboratory test results and the first field trial of the 100% oil system with mineral oil as the base and glass bubbles as the density reducing agent in a drilling fluid. The fluid was used for drilling the 8 1/2" section of MOT-25B well, with an estimated BHT of 250°F. MOT-25B was a deviated well. Drilling with this fluid began at 8275' and reached an Eocene objective at 10,100' (Misoa: B-0/B-4 sand). The density range was 7.1 ppg to 7.3 ppg with 15 ppb using calcium carbonate as bridging agent.
This paper was prepared for presentation at the 1999 SPE Latin American and Caribbean Petroleum Engineering Conference held in Caracas, Venezuela, 21–23April 1999.
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