Bowen盆地北部M煤层气田水平井生产差异特征与主控因素分析

Analysis on the difference of production characteristics and main factors for horizontal well in M coalbed methane field of Northern Bowen Basin

  • 摘要: Bowen盆地北部二叠统煤系地层煤层气资源丰富,但勘探开发程度低,为指导有利开发选区,需厘清已开发煤层气田生产特征差异与主控因素。以M煤层气田实际生产数据为基础,从静、动态资料研究入手,根据开发井高峰产气量和产气曲线形态划分4类产气模式:Ⅰ型,高峰产气量>4万m3/d,产气陡升—陡降—后期平缓递减型,该型产气模式高峰产气量高,高峰稳产期短,为3~7个月,递减阶段早期递减快,年递减速度为30%~35%,后期递减缓慢;Ⅱ型,高峰产气量介于2~4万m3/d,产气陡升—后期缓慢递减型,该型产气模式高峰产气量中等,高峰稳产期相对较长,为10~12个月,递减阶段递减相对缓慢,年递减速度为8%~10%;Ⅲ型,高峰产气量<2万m3/d,产气缓升—后期平稳型,该型产气模式产气相对较低,但产气平稳,无明显递减期;Ⅳ型,高峰产气量<1万m3/d,低产不连续型,该型产气模式产气低或无自然产能。通过动、静态资料综合分析,明确了影响M煤层气田上述产气差异特征主控因素:① 受埋深、构造控制,浅埋藏(<350 m)、高构造区域,煤层渗透性高,又是煤层气解吸、扩散、运聚的主要方向,高峰产气量高,主要分布于气田西部,为Ⅰ型产气模式;随埋藏变深(350~500 m)、构造变低,逐渐过渡为Ⅱ型产气模式,分布于气田中部;当埋深>500 m,煤层渗透性<1×10-15 m2,自然产能低,分布于气田东部,为Ⅳ型产气模式。② 受局部低幅微凸起构造控制,高峰产气量高,仅分布气田东部局部区域,为Ⅰ-Ⅱ型产气模式。③ 受顶板封盖性差和煤层分叉变薄、夹矸的影响,开发井高峰产气量相对较低,分布于气田北部和南部,主要为Ⅲ型和Ⅳ型产气模式。基于上述结论,Bowen盆地北部浅埋藏、高构造斜坡区域仍是新区优先水平井开发的有利区域,但须优化浅层开发井网、井距,制定合理排采制度,避免产气大起大落,力求实现高产与稳产趋势均衡;鉴于煤层埋深>500 m区域,煤层气资源量大,须开展煤层压裂改造增产技术攻关,力争突破较深层低渗煤层气资源开发。

     

    Abstract: The coalbed methane is rich in Permian coal seams of the northern Bowen basin,but the degree of exploration and development is low.In order to select the favorable development area and to clarify the production characteristics and the main influential factors,based on the static and dynamic production data in M coalbed methane gas field,and according to the peak gas rate and gas rate curve of the development well,four gas production types are established.For typeⅠ,the peak gas rate is higher than 4×104 m3/d and the gas rate curve has a sharp rise-steep drop-slow decline trend.The peak gas is high and the peak gas time is short about three to seven months.The early decline phase has a high annual decline rate about 30%-35%,and the decline rate becomes slower in the later decline phase.For typeⅡ,the peak gas rate is 2×104-4×104 m3/d and the gas rate curve has a sharp rise-slow decline trend.The peak gas is medium and the peak gas time is relatively long about ten to twelve months,and the decline is relatively slow with annual decline rate about 8%-10%.For type Ⅲ,the peak gas rate is lower than 2×104 m3/d and the gas rate curve has a slow rise-steady trend.The gas rate is relatively low but steady,so there is no apparent decline.For type Ⅳ,the peak gas rate is lower than 1×104 m3/d with a discontinuous production.Moreover,the main influential factors for the above different production characteristics are specified.Firstly,under the control of coal seam depth and geological structure,the area with relatively shallow depth (smaller than 350 meters) and high permeability is the major region for the migration of desorbed and diffused coalbed methane,and it has a high peak gas rate,mainly in the west of the gas field with gas production type Ⅰ.As the increase of coal seam depth,in the depth range of 350 to 500 meters,at the lower location of the geological structure,the gas production type Ⅱ distributes in the middle of the gas field.When the depth of coal seam is deeper than 500 meters with permeability lower than 1×10-15 m2,the productivity is poor,and it lies in the east of the gas field with gas production type Ⅳ.Secondly,under the control of local low micro-structures,the peak gas rate is high,and the area is only in some region of the eastern gas field with gas production typeⅠandⅡ.Thirdly,with a poor caprock and the split thin or stoned-band coal seam,the area has a relatively lower peak gas rate and it is mainly in the north and south of the gas field with gas production type Ⅲ and Ⅳ.Based on the above analysis,the selected favorable area is located in the slope region with shallow depth and high structure of the northern Bowen basin,and the horizontal well has the priority.Meanwhile,the well pattern and spacing should be optimized with a reasonable production strategy to avoid the big fluctuation of the gas rate and to strive to reach a high and steady production.For the regions with a depth deeper than 500 meters,the gas resource is abundant and the fracturing technology should be carried out aiming for the breakthrough of the development for low permeability coal seams in relatively deep areas.

     

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