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TX 75083-3836, U.S.A., fax 01-972-952-9435.

The problem of lost circulation occurs almost permanently during drilling operations. When drilling highly permeable, highly fractured, and depleted zones, large drilling fluid losses lead to increased operational expenses. Lost circulation may have several consequences, including fluid inflow, wellbore collapse, formation damage, nonproductive rig time, and environmental issues. In the last century, lost circulation has presented great challenges to the petroleum industry, requiring large capital expenditures and time to address the problem. A particular challenge is depleted reservoirs. As the reserves decline, pore pressure decreases, resulting in weakened hydrocarbon-bearing rocks; nearby or inter-bedded low permeability rocks, however, may maintain their pore pressure. This situation can make the drilling of certain depleted zones extremely difficult. Generally, the fluid density necessary to support the shale above exceeds the fracture resistance of the sands, silts, or carbonates below. Consequently, it is vital to design the drilling fluid to minimize mud invasion into formation and prevent lost circulation, and to provide the possibility of removal at a later stage (completion).To mitigate circulation loss risks, a broad range of treatments and preventive methods using lost circulation material (LCM) have been tried and recommended over the years with varying degrees of success and efficiency. This paper presents an extensive review of the applications of various technologies, both proven and potential, in LCM for sandstone and carbonate depleted reservoirs. It describes the state of the art, advantages, limitations, and suggested specific applications. It also presents recent references of their uses in the petroleum industry and discusses the application and the status of the technologies.For sandstone, the materials discussed focus on LCM in the form of flakes, granules, fibers, blends of these materials, particulate materials, polyglycolic acid (PGA) and polylactic acid (PLA) materials, as well as nanoparticles (NP) and nanocomposites. For carbonate formations, the scope includes conventional LCM, such as cement plugs, settable plugs, polyurethane grouting and other new technologies, such as gels, viscoelastic surfactant gels (VES), and crosslinked gels.This review compiles crucial information necessary to design and formulate the fluid using LCM for the depleted reservoir challenges.

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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