Mitigating Gas Hydrate Related Drilling Risks: A Process-Knowledge Management Approach

Abstract

Abstract In order to secure future energy needs, exploration for hydrocarbon reservoirs occurs increasingly in Arctic and Deep Water Environments. These two drilling environments establish temperature-pressure conditions that are favorable to gas hydrates. Gas hydrates are crystalline complexes of water molecules, which trap gas molecules of suitable size inside their lattice structure. If they are created inside the well system, they may block the BOP or lines, which leads to well control risks. When gas hydrates exist naturally inside formations, drilling activities may lead to their dissociation, which can result in borehole stability problems, serious well gasification or subsequent well integrity problems on production. To mitigate these drilling risks a Process-Knowledge Management System (PKMS) has been developed. This PKMS allows the capture, verification, and intelligent use of explicit and tacit knowledge, and is constructed to allow exploitation of the knowledge from multiple field experts, company best practice policies, and latest technological findings. To incorporate and use the knowledge sources with embedded uncertainties, type-2 fuzzy set theory is applied. The PKMS is realized through individual reasoning blocks that set up a coherent reasoning lattice. The benefits of this approach are:An easy adaptation and upgrading of the system's knowledge, andSensitivity for selection of particular reasoning blocks, depending on the current drilling scenario and conditions, is sustained. For the process of drilling within gas hydrate prone environments, the PKMS is actuated with multiple numerical simulations and gas hydrate related considerations. This paper discusses briefly the basic structure of the developed PKMS, as well as the numerical models used to describe the transient temperature field, the pressure profile and the gas hydrate kinetics applied. Introduction To search for new hydrocarbon reservoirs and to exploit already verified ones, drilling is carried out increasingly in Deep Water and Arctic locations. Both of these drilling locations establish pressure-temperature conditions such that solid gas hydrates are stable. Gas hydrates may exist naturally inside the formations drilled, these are called natural gas hydrates, or they can form inside the well system. Gas hydrates are crystalline complexes of water molecules that trap gas molecules of suitable size inside their lattice structure. Depending on the type, mixture and size of the molecules bound inside the gas hydrate, as well as the pressure-temperature conditions present, three different gas hydrate structures are possible. They are namely: structure I (sI), structure II (sII), and structure H (sH). Sloan1 gives detailed descriptions of gas hydrate forming conditions, and their crystal parameters. The potential risks and well problems to be considered due to the presence of gas hydrate while drilling are discussed by Prassl2. For initial assessment of these risks, the potential existence of solid gas hydrates at the particular well location is needed. For this, a superposition of the gas hydrate stability curve and the natural temperature gradient is carried out usually, see figure 1. Within this study, gas hydrate formation and dissociation are assumed to occur at the same pressure-temperature conditions, i.e. the hysteretic behavior of them is neglected. Hence, the gas hydrate stability curve applicable is defined by:Gas composition that forms gas hydrates;Formation water salinity (salts act as thermodynamic inhibitors);Formation pressure profile and pressure profiles inside the well;Formation temperature profile and temperature profiles inside the well; andSediment/gas hydrate and mud component/gas hydrate interaction mechanism. Note that these parameters are associated with various degrees of uncertainty.