Experience Using Microbubbles-Aphron Drilling Fluid in Mature Reservoirs of Lake Maracaibo

Abstract

Abstract 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.