Ultralightweight Cementing Technology Sets World's Record for Liner Cementing With a 5.4 lb/gal Slurry Density

Abstract

Abstract In an effort to meet increased demand for petroleum products, operators are drilling in more challenging environments. In Mexico, meeting the new challenges is requiring more skills and innovative materials from oilwell cementing providers. Nowhere else in the world have service providers attempted cementing across more depleted zones or weaker formations. Current cementing technology may not ensure successful circulation and an effective seal, which can have a great impact on (1) the cost of drilling a well and (2) the productive life of the well. In addition to the easily quantified cost of often-required remedial work, additional costs that are more difficult to quantify can be incurred from delays in production delivery and early water production. Traditionally, the industry has relied on two very different technologies for the creation of ultra-light (densities < 10 lb/gal) cement slurries. The current technology involves either (1) the creation of a foamed cement using nitrogen or (2) the addition of lightweight microspheres (LMS) (sg < 1.1) into the cement. While both of these technologies are capable of producing very light cement slurries, neither of them has been used in an oil well to form an effective seal at densities below 7.3 lb/gal. In these most difficult Mexican formations where fracture gradients range from 7.3 down to 6 lb/gal, current cement densities of 7.3 lb/gal or greater have reduced chances of successful circulation. This paper presents case histories from Mexico showing how a melding of both of these technologies, foam and microspheres, were required to stretch current lower slurry-density limits, allowing us to meet extreme operational demands. Along with the case histories and laboratory data, guidelines are provided for the best use of these technologies at densities down to 5 lb/gal. Details provided from the case histories illustrate how the advantages of both technologies were maximized, while their limitations were minimized. Background Standard density for oilfield cement is 15.6 to 16.4 lb/gal. If wellbores encounter weak or depleted zones, standard cement cannot be used because the bottomhole circulating pressure (BHCP) will exceed the rock strength, open fractures in the rock, and get lost to the formation. If a portion of the cement is lost to the formation, top of cement (TOC) will be reduced. If the TOC is significantly lower than designed, remedial cement jobs may be required, driving up the well construction costs and wasting valuable drilling days. To avoid losing cement to the formation, the cement density must be reduced to a level where the hydrostatic pressure plus the frictional forces are less the fracture gradient. In less difficult drilling conditions, adding extra water (and necessary water-extending additives that will tie up the extra water) can reduce the slurry density sufficiently to allow the cement to be circulated, reaching the desired TOC. As additional water is added above the standard 4 to 6 gal/sk the cement becomes more dilute, strength declines, and permeability increases. As the density decrease approaches 11 lb/gal, the dilution affect becomes so great that compressive strength development is too slow or so low that it becomes impractical for use in oil or gas wells. In wells that require densities <11 lb/gal or higher quality cement at densities >11 lb/gal, the industry has turned to use of LMS or stable foam for density reduction.[1,2] Both of these alternative systems have advantages compared to water-extended systems and when compared to each other. In the LMS method, lightweight microspheres composed of relatively inert materials are added to the cement slurry. These LMS are typically pozzolanic or glass and hollow or a more elastic solid material. The key advantages of the LMS systems include:the capability to create competent cement down to 7.3 lb/gal,fastest strength development of any system at a given density (meaning a reduced waiting on cement (WOC) time),the lowest permeabilities at a given density, andthe capability to use conventional mixing equipment. The second premium density reduction method involves creation of a stable foam. The preferred practice calls for the injection of nitrogen through a properly sized choke in conjunction with special chemicals to stabilize the foam. Advantages of foamed cement include:has the capability to create a high-quality, low-density cement slurry,increases set cement elasticity,increases liquid-state compressibility, andcan be very cost effective.