Research on Waterless Fracturing Technology in Tight Gas Formation in China

摘要

Hydraulic fracturing was an important means for tight gas reservoir development in an attempt to achieve commercial production and to improve productivity.For fracturing job in low-pressure and tight gas reservoir,it is vital to reduce invasion by fluids and improve fluid flowing back,which directly affect the individual-well performance.Conventional water-based fracturing technology was apt to cause problems such as difficulty to flow back for fracturing fluid and serious water lock damage,hence,in this paper,CO2 foam hydraulic fracturing techniques was studied to manage the plaguy trouble.Firstly,the widespread using guar gum can cross-linked and keep stable viscosity only under the condition of the PH(hydrogen ion concentration)>7,however,the CO2 solution PH is 4 to 5.So,novel acid CO2 foam fracturing fluids were developed based on the study of high temperature crosses linker and Environmentally friendly foaming agent,which efficiently resolved the cross-linking in acid conditions,also the additives are environment friendly and low cost to accommodate the low oil price.Systematically tests indicated that the fluid possessed the advantage of improved rheological characteristic,good proppant suspension ability,low friction and thoroughly and rapidly gel breaking without residues.The fluids used a few additives and were easy to prepare on construction site.Secondly,for the deep-buried reservoir,the pipe friction is large especially for CO2 fracturing fluid owing to its complicated phase state,whereas the treating pressure is limited,which induce the displacement couldn't be increased to the required value.So,the formula for calculating pipe friction was deducted in detail to forecast wellhead pressure,and the field experimental results proved its correctness.Then,pumping string and injection scheme were optimized to reduce operating pressure and guarantee the construction success,thirdly,in order to enhance the amount of adding proppants,the optimization design of variable foam quality and variable flow rate were adopted in according to the reservoir conditions,finally,extraordinary fracturing design,integrating ordinary fracturing design and tip screen-out(TSO),were employed to improve the artificial fracture conductivity,under flush technique was applied to keep the near-wellbore fracture conductivity.Field application showed that,using CO2 foam fracturing technology,a large amount of sand fracturing treatment was carried out successfully in vertical wells in Sulige gas field at the depth of 3400 m in china,the flow back of gel breaking liquid was rapid and flow completely without pumping.The average daily gas output of experimental well is 2.2 times that of the same geological conditions of adjacent wells treated by traditional water-based fracturing.It shows effectiveness of the optimization design method and good application prospect of CO2 foam fracturing in low pressure tight gas reservoir.