Research on intelligent judgment method of natural gas hydrate drilling risk

There are many emergency risks in the process of natural gas hydrate(NGH)drilling.In order to ensure the safe and efficient exploitation of NGH,it is urgent to establish an intelligent judgment method for the risks in the process of NGH drilling.In this paper,the response relationship between monitoring parameters and risk categories of NGH while drilling is established.Based on fuzzy analytic hierarchy process(FAHP),the comprehensive weights of 10 risk monitoring parameters are obtained,including gas production,wellbore instability,hydrate ice barrier,drill string fracture,sticking,bit balling,drilling tool piercement,gas seepage,seabed subsidence and seabed landslide.Besides,the comprehensive judgment weight matrix is constructed,and the reasonable fluctuation range of monitoring parameters is formed.Thus,the intelligent judgment method of NGH drilling risk is established.The intelligent judgment and alarm of NGH drilling risks can be realized quickly and accurately by this method,namely,it can monitor the risks in the process of operation and guarantee the construction safety of NGH drilling.

Alcohols for hydrate inhibition-Different alcohols and different mechanisms

Methanol has been used to prevent hydrate formation in industrial handling of hydrate forming mixtures containing water for many decades.Ethanol is also used for the same purpose in countries that have easy access to low price ethanol,like for instance Brasil.Common to these small alcohols is that they also have surfactant properties that will promote hydrate formation,but when added to water in sufficient amounts,the hydrate prevention characteristics will dominate.These alcohols will primarily prevent heterogeneous hydrate formation on the interface between water and a separate hydrate phase.The effect of“alcohol”on both of these routes to hydrate formation are investigated and compared to experimental data.In particular we also investigate the effects of these small alcohols on Gibbs free energy for the hydrate formed on the new,shifted,stability conditions.Gibbs free energy is generally higher than hydrate formed from pure water.Enthalpies of hydrate formation are also higher for hydrate formed from water containing alcohols.These are negative numbers,so in absolute values released formation enthalpy is lower.The presence of these alcohols in water will also prevent homogeneous hydrate formation from dissolved hydrate formers in water.Glycols have more important roles in other routes to hydrate nucleation.Heterogeneous hydrate nucleation towards mineral surfaces is feasible in different ways.Polar hydrate formers like H2S and CO_(2)can adsorb directly on rust,and as discussed here,are able to form hydrate from adsorbed state on rust surface.Non-polar hydrocarbons like,for instance methane might get trapped in structured water and then nucleate to hydrate.Some research on this is published and further research is in progress.Glycols have very strong attraction to rust and corresponding chemical potentials for adsorbed glycols on rust are favourable enough to facilitate phase transition from glycols dissolved in water over to adsorption.Injection of glycol in gas processing plants has been used by industry for many years and in many cases it might even be economically and technically feasible compared to expensive drying units.Exceptions are situations that will lead to water/glycol freezing.But even in multiphase transport of hydrocarbons with various water cuts,mixtures of alcohols might be a technically efficient solution in which the small alcohols may be very efficient as discussed above and glycols may go through adsorption phase transition from water solution over to glycol film on rust and prevent hydrate nucleation towards rust surface.This possible strategy requires more theoretical work as well as experimental investigation.On the basis of thermodynamic analysis and calculations of hydrate formation from different routes,it is argued that real natural and industrial systems are unable to reach thermodynamic equilibrium.It is therefore a need for a consistent thermodynamic platform with a uniform reference system for all phases.We propose and demonstrate a residual thermodynamic model system for all phases.

Sensitivity analysis of reservoir risk in marine gas hydrate drilling

It is easy to change the original temperature state of marine gas hydrate reservoir by drilling,which leads to uncontrollable decomposition of gas hydrate and release of large amount of gas.The decomposition gas will further escape and expand,and the reservoir will break and collapse due to its weak cementation characteristic,which will easily lead to a series of other potential risks.Therefore,in this study,based on the drilling process of marine gas hydrate,we establish the theoretical model and numerical calculation method of wellbore temperature field,analyze the influence on wellbore temperature of drilling fluid displacement,density,viscosity and injection temperature,and seawater depth.Then the sensitivity laws of reservoir risk in marine gas hydrate drilling are obtained.The results show that with the increase of drilling fluid displacement,density,viscosity and injection temperature,the temperature in lower well section and bottom hole will increase,making the increasing amplitude of temperature in hydrate reservoir larger and the scope of influence on hydrate reservoir stability bigger.Moreover,drilling is more likely to raise the temperature of reservoirs in shallow seawater depth,posing greater risks.Thus,engineering measures to avoid risks caused by rising reservoir temperature in marine gas hydrate reservoir drilling are presented.This study is of great significance to ensure the safety of marine gas hydrate reservoir drilling.

Routes to hydrate formation from water dissolved in gas and impact of mineral surfaces

Mineral surfaces adsorb water to extreme densities and corresponding low chemical potentials.This results in a dual effect in terms of hydrate.Water and slightly polar components adsorb directly on mineral surfaces and generate efficient conditions for hydrate nucleation.But due to the extremely low chemical potential of adsorbed water the hydrate nuclei formed towards mineral surfaces have to either detach from the vicinity of mineral surfaces,or be bridged by structured water in a dynamic attachment of hydrate cores some few nm outside mineral surfaces.During transport of gas(CH4,gas mixtures,CO2)the conventional water dew-point analysis will typically result in a substantially higher acceptable water concentration as compared to the concentration for adsorption of water from gas to rust surface.Direct formation of hydrate from water dissolved in gas is thermodynamically feasible,as discussed in open literature.In this work we demonstrate that it is also feasible in terms of mass transport.A new theory for enthalpy of hydrate dissociation has been extended to also direct hydrate formation from water dissolved in gas.The remaining question is whether direct hydrate formation from gas is also feasible in terms of transporting the hydrate formation heat away through a heat insulating medium.We propose further research strategies to enlighten this issue.Addition of glycols to critical points in processing of gas or transport is already in use by companies like for instance EQUINOR.There is,however,a need for more work on how efficient it is and if it can also be used for multiphase transport of hydrocarbons with significant water cut.Some research activities are in progress and briefly outlined here.