Depleted reservoirs pose numerous technical challenges in both the construction and completion phases for wells in dozens of producing fields, often putting into question the economical viability of these fields. Wellbore instability, severe lost circulation, and stuck pipe are just a few of the problems encountered when drilling into these low-pressured reservoir formations.No area better illustrates the problems with depleted reservoirs than the Lake Maracaibo region. Water-wet sands that frequently triggered costly seepage losses and differential sticking typify many of these zones. Some contain microfractured sandstone formations where uncontrollable losses of whole drilling fluid previously were the norm rather than the exception. Others are characterized by laminated sand and shale sequences, which create the conditions for slow, dangerous, and unduly expensive drilling. Attempts were made with underbalanced drilling, but in addition to the extra time and equipment required, wellbore instability lead to failed well construction and thus seriously degrading project economics.Over the past two years, a specialized drilling fluid has being utilized to drill these depleted reservoirs in Lake Maracaibo. This fluid combines certain surfactants and polymers to create a system of "micro-bubbles" known as aphrons encapsulated in a uniquely viscosified system. These aphrons are non-coalescing, therefore creating a micro-bubble network for stopping or slowing the entry of fluids into the formation.The aphrons allow conventional drilling equipment to be used to successfully complete many reservoirs that previously would have been candidates for underbalanced drilling only. This paper describes the development and application of the specialized "micro-bubbles" or aphron-based drilling fluid for drilling depleted reservoirs by controlling downhole mud loss and formation damage. The authors will detail the operational procedures and the field applications of this drilling fluid, with particular emphasis on the lessons learned in the Lake Maracaibo implementation of the system.

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.

Improved drilling techniques have overcome many of the inherent difficulties associated with the delivery of Icotea and Misoa wells in the West Urdaneta Field of Lake Maracaibo. Advancements such as rotary steerable assemblies, logging-while-drilling (LWD) tools, annular pressure subs and new bit designs have allowed drilling of extended reach and horizontal wells to become routine. However, borehole instability and lost circulation problems have continued to negatively impact operations, compounding well delivery costs. In an effort to address these problems, a variety of non-aqueous fluids (NAF) and water-based muds (WBM) have been used. While each provides distinct advantages, there has never been a system which could solve all problems associated with drilling the Icotea and Misoa formations. Lost circulation and wellbore instability (defined by almost continual caving of shales in the La Rosa formation) have plagued NAF applications in this field. A variety of WBM systems have been used and each one has also presented its own problems including hole enlargement, bit balling, accretion, low rates of penetration, insufficient hole cleaning, and the need for excessive backreaming. Finding a fluid that would deliver step-change performance in of drilling efficiency compared to previously used systems, while also adhering to the strict environmental limitations for the area, was paramount to continue economically viable drilling operations in the Urdaneta Field. This challenge was met using an environmentally benign, low-salinity high-performance water-based mud (HPWBM), which was field tested in the intermediate section of the Icotea and Misoa wells. The novel fluid was used to drill through the problematic Laguna, Lagunillas, and La Rosa formations. By analyzing the field tests results, conducting after action reviews (AAR) and maintaining a productive dialogue between all parties involved, the operational performance of these wells showed substantial and continual improvement. The low-salinity HPWBM is now our system of choice for drilling intermediate hole sections in the West Urdaneta Field. This paper provides a detailed technical overview of the new HPWBM, provides and the results of the early field tests and of subsequent wells, demonstrates enabling impact of this new technology on driving continual operational improvement. Introduction As the search for new oil and gas reserves continues, operators are increasingly moving towards drilling more challenging well trajectories. The search for new oil reserves has also led operators into more remote and environmentally sensitive areas. Drilling in these areas, especially with complex well designs, presents considerable technical, economical, and environmental risks and challenges. The intermediate sections of West Urdaneta wells have been drilled with a wide variety of WBM systems since the first well was drilled in 2001. Dispersed WBM, at first, yielded high rates of penetration (ROP) However, these systems were plagued by excessive hole enlargement, clay instability, problematic trips, pack-off and lost circulation events. More inhibitive WBM's were tried and while these systems produced satisfactory clay- and shale stability, problems arose from accretion and associated ROP reduction. Finally, attempts were made to improve fluid performance through use of NAF. The continual generation of cavings (creating problems with hole cleaning, and tripping pipe) and catastrophic lost circulation events that occurred with these fluids more than negated the benefits they provided with respect to reducing accretion and increasing ROP. Legislation to control the use and discharge of cuttings and spent fluid in Lake Maracaibo area is becoming more stringent. The Venezuela Ministry of the Environment has developed legislation to minimize the impact of drilling activities by eliminating discharges to Lake Maracaibo, not only of NAF, but also of the high salinity WBM.

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The use of managed pressure drilling (MPD) during drilling operations has proven to be an effective solution for controlling equivalent circulation density (ECD); however, making a drillpipe connection with MPD is more challenging than with conventional drilling. The current methods for making a connection during MPD rely on large, high-maintenance equipment to provide a sufficient fluid supply to maintain the MPD equipment within an operable range. This equipment is both expensive and difficult to use to control ECD efficiently. A new method of smoothly diverting rig pump flow during connections from the stand pipe to the MPD pressure control equipment at the annulus uses a valve manifold with an on-board choke. In addition to eliminating more expensive solutions, this method will improve pressure control and increase the operating range of MPD to provide higher pressures during connections and greater drilling flow rates. Reliability is increased by a reduction in complexity when using this method, because most rigs have multiple pumps available for redundancy. Field trials using a fully automated MPD solution were performed by a major operator in South Texas between late 2010 and early 2011. This paper uses real data based on flow testing and actual field experience to demonstrate that implementing the rig pump diverter method with standard MPD operations effectively controlled the desired annulus pressure during connections. By replacing current methods of making a drillpipe connection, this solution will reduce the overall cost of MPD, making the total MPD solution more feasible for small wells.