| At present,it is difficult to comprehensively evaluate the level of environmental risks in the whole natural gas exploitation process because there are many and complicated environmental risk factors involved in the pre-drilling,drilling and post-drilling technological processes,and the environmental risks are uncertain.So,in order to solve that problem,this paper based on the characteristics of natural gas exploitation technology to analyzes from the whole process of natural gas exploitation,and constructs an index system for evaluating the environmental risks of the whole process of natural gas exploitation,which provides some ideas and decision-making basis for the natural gas exploitation industry to comprehensively evaluate the environmental risks.The system includes one target layer and three criteria layers,of which the criteria layer is divided into 4 first-level indicators,14 second-level indicators and 41 third-level indicators.Then the weight of each index is determined by AHP.Among them: The weight values of the first-level indicators "pre-drilling engineering risk","drilling engineering risk","completion engineering risk" and "lack of human management" are 0.3852,0.3121,0.1463 and 0.1564 respectively.(1)The weight values of the secondary indicators "well site selection","well site layout safety","new pool body" and "blowout pit" under the "pre-drilling engineering risk" index are0.4117,0.3270,0.1190 and 0.1423 respectively;(2)The weight values of the secondary indexes "drilling","well control","completion of drilling","logging and mud logging" and "drilling fluid" under the "drilling engineering risk" index are: 0.3091,0.2596,0.1085,0.1633 and 0.1595 respectively;(3)The weight values of the secondary indexes "cementing","gas test risk" and "risk after evacuation" under the "completion engineering risk" index are: 0.4395,0.3967 and 0.1638 respectively;(4)The weight values of the secondary indicators "employee operation safety" and "enterprise emergency management deficiency" under the indicator of "human management deficiency" are 0.5652 and 0.4348 respectively.The three-level index "distribution of environmentally sensitive targets","distance from environmentally sensitive points" and "hydrogeological conditions" have 41 index weights.according to the overall ranking of levels,the important ranking of the first-level indicators relative to the target layer is:pre-drilling engineering risk B1>drilling engineering risk B2>human management deficiency B4>completion engineering risk B3;The order of importance of secondary indicators relative to the target layer is C1> C2> C5> C13>C6>C14>C10>C11>C4>C8>C9>C3>C7>C12.Finally,according to the principle of fuzzy comprehensive evaluation method,a fuzzy comprehensive evaluation model for environment risk assessment in the whole process of natural gas exploitation is established.The expert investigation method is used to evaluate the indicators step by step with the "lower,medium and higher" evaluation set combined with cases,and the principle of maximum membership degree is used to make comprehensive analysis.It is concluded that the comprehensive evaluation result of environmental risk assessment for a natural gas well in Liangping District is X=max(0.342,0.564,0.094)=0.564,which is "medium",indicating that the environmental risk is general,and some indicators have relatively large risks,and preventive measures should be taken for links with relatively large risks.Combined with actual cases,the following suggestions are put forward: reasonably selecting well pads,optimizing well pad equipment layout and avoiding economic risks;Strengthen skills training,standardize technical operation of employees,and carry out safety education for employees;Take corresponding preventive measures against environmental risks during the whole process of natural gas exploitation.At the same time,environmental risk prevention and management measures in the whole process of natural gas exploitation are put forward. |
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