Research On The Fracturing Damage And Countermeasures Of Tight Gas Reservoirs

Being rich in natural resources, China has abundant tight gas reserves. But exploration of tight gas is still in early age. Cumulative proved reserves of tight gas has been ascertained to 3.3×1012m3 by late 2011, occupying 40% of total proved natural gas domestically, and recoverable reserves share one third of total natural gas, which is 1.8×1012m3.Due to the low porosity and permeability nature of tight gas, regular techniques are hard to be applied, and fracturing technique is used instead to enhance gas recovery. However, series of causes, including improper fracturing fluids, poor fracturing designs and unsuitable flowback measures, contribute to formation damages, lowering permeability and affecting flow capacity of gas to well bore. As a result, fracturing effect is not as well as expected because of decreased recovery efficiency owing to reduced gas production.In this thesis, geological and exploitation characteristics of tight gas reservoir are described. For different locations of damages caused by fracturing, they can be classified to fracturing damage, filter cake damage and formation damage by analyzing damage mechanisms of tight gas. By analyzing causes and effects of different damage, it is recommended that formation damage has the features of permanency, particularity and universality. Factors resulting in formation damages include 5 types:(1) clay swelling and movable;(2) sedimentary damage;(3) wetting conversion;(4) Jiamin action;(5) water lock effect.Based on tight gas reservoir in Western Sichuan Depression, a 3D model was established monitoring hydraulic fracturing. According to damage types and influence factors, single phase flow which is harmless and two-phase flow that do harm were adopted to simulate effects of dimensionless conductivity, which is 0.1, 1, 10 and 100, on gas production. Similarly, effect of filter cake damage on reservoir under different permeability of 0.05 mD, 0.01 mD and 0.001 mD was studied. For water trapping reservoir, monitor gas production under different leak off rate of 1000m3, 3000m3 and 5000m3, and varies shut-in time of 1-day, 5-day, 10-day, 30-day and 100-day. Finally, by monitoring different pressure drop of 5MPa、15MPand 22.5MPa, study the effects on gas production.Results illustrate that gas production of non-pollution wells are higher than the pollution one. Larger conductivity, more gas production and better fracturing effects will get. Longer shut-in time, more residual fluids in formation, damages are more serious, and effects on production wells getting worse. More flow back pressure drops do efficient to dewatering, water saturation decreasing as a result, and increasing relative permeability of gas, which result to a better production.In view of the water phase trap damage to the reservoir, by adding 1% concentration of surfactant, this paper simulates the different shut-in time 1 day, 5 days, 30 days and 100 days, 10 years of accumulated gas well production, and it is concluded that under the different fracture diverting capacity of surfactants increase yield difference.Therefore, suggestions are provided as follows: firstly, proper fracture fluid should be used to lower the damage and to avoid retaining in fracture face and nearwell bore. Secondly, we should apply suitable flow back measures according to different situations to improve flow back efficiency and avoid damage, thus the gas production after fracturing will be better. Additionally, surfactant can be employed to change relative permeability of gas, decreasing formation damage caused by water trapping, getting better production benefits.