Abstract
Abstract
In the past few years, oil companies have ventured more frequently into deepwater areas in the search for hydrocarbons. This trend is likely to continue, or even accelerate, in the next few years and, if anything, we are likely to see ever-greater water depths.
There are several aspects of deepwater wells that make them particularly challenging, from an engineering perspective, through all phases of the process - construction, production, intervention and abandonment. This paper, however, focuses on the issues surrounding deepwater well cementing, primarily during the well construction phase.
This has been an area of intense interest in recent years, due to industry realisation that the deepwater environment had one or two surprises for drillers. Most notable amongst these is the problem of shallow water flows that can easily wash out the weak, unconsolidated sediments, resulting in seabed subsidence and loss of the hole. Other issues include the presence of shallow gas and gas hydrates, strong subsea currents and extremely low fracture gradients, with the everpresent risk of lost circulation or wellbore collapse. The low seabed temperature, which can be below freezing, also depresses the normal geothermal gradient to a variable depth, depending on the thermal properties of the strata.
None of this is good news for cement, which is required to have short thickening time, rapid development of mechanical properties, a fast liquid:solid transition and low permeability to provide casing support, cope with the risk of influx and provide a long term hydraulic seal, amongst other things. This has driven the development and marketing of a host of proprietary cementing systems that claim to address some, or all, of these problems.
This paper reviews the deepwater cementing issues, in detail, and examines the physical and mechanical properties of various cement systems to assess which parameters are truly critical to success. It combines laboratory data with field case histories and working practices in several parts of the world, to help engineers decide on the best formulations for cementing deepwater wells.
Introduction
Although the first deepwater wells were drilled in the 1970's, it is only recently that there has been a widespread focus on exploration and development in this challenging environment. Yet, in a relatively short span of years, even the very term "deepwater" has evolved. The generally-accepted meaning of the term used to be "water depths over 1000 ft" but, more recently, it has come to be regarded as "water depths in excess of 500 m (1640 ft)", a substantial increase in depth. Irrespective of the depth threshold applied, the excitement generated from hydrocarbon reservoirs in deepwater environments stems largely from the fact that many of these sedimentary sequences are very young, geologically. As a result, they have undergone only limited compaction and, typically, have very high porosity and permeability, characteristics that are favourable for reservoir rocks since they imply large potential reserves and high productive capacity. Recent estimates of hydrocarbon reserves discovered, to date, in deepwater amount to 57 billion barrels of oil equivalent (BBOE), comprising 37 billion bbls of oil and 120 tcf of gas. Similar estimates of, as yet, undiscovered hydrocarbons suggest that a further 85–100 BBOE probably still lie in deepwater reservoirs.
However, the very features that make these deposits interesting also pose major challenges for those trying to drill and exploit them, as discussed below.