Study on the blockage in pores due to asphaltene precipitation during different CO2 flooding schemes with NMR technique

CO₂ flooding is an effective way in the tertiary oil recovery. While asphaltene often precipitates from the crude oil during the CO₂ flooding, and the mechanisms of blockage resulting from asphaltene precipitation is still unclear in different CO₂ flooding schemes. In this work, pure-CO₂ flooding, water-alternating-CO₂ flooding (WAG), and CO₂-foam flooding were applied to conduct the core-flooding experiments. Then, as for each flooding scheme, we quantitatively investigated the blockage degree in different pores due to asphaltene precipitation with nuclear magnetic resonance (NMR) technique. Tests results show that CO₂-foam flooding has a relatively higher blockage degree both in the smaller pores and the larger pores than WAG and pure-CO₂ flooding. Although pure-CO₂ flooding has the least asphaltene precipitation and blockage degree among three flooding schemes, its oil recovery degree is far less than the other two flooding schemes. Compared with pure-CO₂ flooding and CO₂-foam flooding, WAG flooding has the highest oil recovery and an acceptable asphaltene precipitation.     

水气交替注入对CO_2驱油效果的影响

延长油田靖边203区块是典型的低孔低渗油藏,该区块正在进行CO_2驱先导性试验。水气交替注入(WAG)能有效控制驱替前缘流度,抑制气窜并延长见气时间,从而改善CO_2驱油效果。通过与连续气驱对比实验,研究了WAG对CO_2驱油效果的影响,并探讨了注气速度、段塞尺寸和气水比等注入参数对CO_2驱油效果的影响规律。结果表明:对于非均质性油藏,WAG比连续气驱具有更明显的优势;渗透率级差为10~30时,WAG能有效地抑制气窜,改善驱油效果;渗透率级差过大或存在裂缝时,WAG驱油效果变差;WAG的注气速度、段塞尺寸和气水比分别为50 m L/min,0.10 PV和1∶1时,驱油效果最佳,采出程度在水驱基础上提高了20.95百分点,最终采出程度为44.70%。     

井间示踪技术在大庆油田的应用

本介绍了应用井间示踪技术成功地研究了水气交替注入后，大幅度提高平面波及效率，在厚油层中产生重力分异，驱挪了水驱波及不到的地区提高采收率的机理，并在油田的调整挖潜，提高原理产量和试验效果方面起了重要作用。     

聚合物与水气交替联驱提高采收率试验研究

采用三种不同沉积韵律平面模型试验研究水驱后聚合物与水气交替联驱提高原油采收率的驱替特征。结果表明,聚合物与水气交替联驱对正韵律油藏提高采收率效果很好;对反韵律油藏和复合韵律油藏也有不同程度提高采收率效果。通过分析两种不同韵律砂岩油藏水气交替提高采收率矿场试验成功实例,认为聚合物与水气交替联驱法可综合发挥两种方法的优势,在类似油藏中比单一方法有更好的提高采收率效果。     

Comparative Study of Hysteresis Models in Immiscible Water Alternating Gas (WAG) Process

Abstract

To simulate multiphase flow in complex recovery processes such as water alternating gas (WAG), it is essential to consider the influences of saturation history on relative permeabilities and capillary pressures of the present phases. This effect, which is known as hysteresis, is generally handled by use of empirical models, and deciding which combinations of hysteresis options and parameters are appropriate for a specific study is required.

This study investigates how reservoir simulation results are influenced by the use of different modeling options for hysteresis effect in three-phase flow. Hysteresis models were categorized for wetting and nonwetting phases, and they were applied to relative permeability and capillary pressures. In addition, sensitivity analysis was performed on effective parameters on hysteresis behavior of relative permeability.

Laboratory hysteresis data were used in simulation of an immiscible WAG process. It was observed that including hysteresis options in simulation influenced the outputs significantly, mainly on oil recovery and breakthrough times (water and gas). Changing the type of nonwetting model had a negligible influence on the results, whereas the type of wetting hysteresis option had a considerable effect on prediction of recovery and breakthrough times. Furthermore, using hysteresis, capillary pressure was found to have a very weak effect on the outputs. Finally, it was observed that the significance of the hysteresis effect is very sensitive to imbibition residual saturation of oil in the presence of water.     

Experimental Investigation of Miscible CO2 Flooding

Abstract

Management of water alternating gas (WAG) injection projects requires making decisions regarding the WAG ratio, half-cycle-slug size, and ultimate solvent slug size. The impact of these decisions affects the capital cost and ultimate incremental oil recovery. Core flooding runs were conducted on 2 and 4 ft core samples. Injection scheme (continuous gas injection [CGI] vs. WAG), WAG ratio, and slug size were investigated. In addition, miscible WAG flooding as a secondary process was investigated and its efficiency was compared to the conventional tertiary miscible gas flooding. Miscible gas flooding at different miscible WAG parameters (WAG ratio and slug size) indicate that 1:2 WAG ratio at 0.2 PV slug size is the best combination yielding the highest recovery and tertiary recovery factors. Miscible WAG flooding as a secondary process indicated a higher ultimate recovery compared to the conventional tertiary WAG flooding. However, a larger amount of gas injection is consumed particularly in the early stages of the injection process. Miscible CGI mode conducted using n-Decane as oleic phase appears to have better performance than miscible WAG injection in term of recovery. When light Arab crude oil was used as oleic phase, higher recovery was obtained for miscible WAG flooding. The reversal trend seen in is believed to be due to the presence of crude oil, which alters the rock wettability toward an oil-wet condition, preventing the water blockage during the WAG process.     

The Injection Pattern of Immiscible Nitrogen Displacement After Water Flooding in an Ultra-low Permeability Reservoir

With the nature ultra-low permeable cores from Changqing Oilfield, the simulated experiments of nitrogen drive after water flooding have been accomplished for the ultra–low-permeability reservoir. The injection pattern of nitrogen includes direct gas injection, water/gas alternating injection and pulse gas injection. The results show that direct injection could not develop the ability that nitrogen enters into the local tight area to improve the microsweep efficiency, because the good mobility generates gas channeling in some local high-permeable zone. Subsequent direct nitrogen injection after water drive only enhances the oil recovery ratio by 3.65%. The dispersion degree of oil, water, and gas is much higher for water/gas alternating injection after water injection, and the capillary force is the main resistance to suppress the rapid gas flow validly in local high-permeable zone. Nitrogen is enforced to drive the residual oil in local tight area or micro pore system. Water/gas alternating injection after water flooding enhances oil recovery ratio by 16.37%. Pulsing injection can take advantage of the elastic energy of the high-pressure nitrogen and pressure disturbance effect, which makes pressure redistribution and fluid deallocation between local high-permeability zone and local compact area. With pulse injection, oil in local tight region also can be driven by nitrogen, and the recovery ratio is enhanced by 15.94%.     

QK17-2油田注N2-WAG提高采收率可行性实验研究

气水交替驱(WAG)是增加油田水驱后的波及体积和减弱气驱过程因油气黏度差异而产生的气体指进现象的有效方法。QK17-2油田为注水开发油田,目前油田综合含水率已超过80%,已经进入开发中后期的高含水时期。针对QK17-2油田水驱后面临高含水、产油量下降的问题,通过地层流体相态实验、注N2膨胀实验、细管实验、长岩心驱替实验,研究注水后再注N2-WAG可进一步提高采收率的可行性。结果表明,单纯注N2效果不佳,注N2-WAG采收率较单纯水驱采收率提高4.46%~6.24%,而长岩心驱替在带倾角条件下注N2-WAG采收率较单纯水驱提高11%~12.58%,可见由油藏高点注N2-WAG采收率将有明显提高。     

低渗透非均质性油藏扩大波及体积技术

为了有效利用水气交替技术提高低渗透、非均质性油藏的采收率,引入注入速度差异控制(AOIR)、段塞比变化的水气交替驱(TWAG)和AOIR-TWAG技术研究了提高采收率的可行性.结果表明,渗透率变异系数越大,水气交替驱相对于连续注气驱的效果越好,3种技术的采收率均高于常规的水气交替驱.但是,如果考虑经济效益,则只有AOIR和AOIR-TWAG技术的净现值高于常规的水气交替驱.通过新疆油田某油藏的模拟验证了概念模型研究的可行性,对提高低渗透、非均质性油藏气驱波及体积具有指导意义.     

An Experimental Study on the Applicability of Water-alternating-CO2 Injection in the Secondary and Tertiary Recovery in One Iranian Reservoir

Abstract

The objective of this study was to experimentally investigate the performance of water-alternating gas (WAG) injection in one of Iran's oil reservoirs that encountered a severe pressure drop in recent years. Because one of the most appropriate studies to evaluate the reservoir occurs generally on rock cores taken from the reservoir, core samples drilled out of the reservoir's rock matrix were used for alternating injection of water and gas. In the experiments, the fluid system consisted of reservoir dead oil, live oil, CO₂, and synthetic brine; the porous media were a number of carbonate cores chosen from the oilfield from which the oil samples had been taken. All coreflood experiments were conducted using live (recombined) oil at 1,700 psi and reservoir temperature of 115°F. A total of four displacement experiments were performed in the core, including two experiments on secondary WAG injection and others on the tertiary water and gas invaded zones WAG injections. Prior to each test porosity and permeability of dried cores were calculated then 100% water-saturated cores were oil-flooded to obtain connate water saturation. Therefore, all coreflooding tests started with the samples at irreducible water saturation. Parameters such as oil recovery factor, water cut, and gas-oil ratio and production pressure of the core were recorded for each test. The most similar experimental work with the main reservoir condition, indicated that approximately 64% oil were recovered after 1 pore volume of WAG process at 136,000 ppm brine salinity. Although tests show ultimate recovery of 79% and 55% for secondary and tertiary injection in gas and water invaded zones, respectively, immiscible WAG injection efficiency in the gas and water invaded zones will not be proper. In the similar test to field properties, the average pressure difference about 70 Psig was observed, which shows stable front displacement. These experiments showed that there was significant improvement in the oil recovery for alternating injection of water and CO₂, especially in the secondary recovery process. Water breakthrough time in almost all of the tests shows frontal displacement of injected fluid in cores and produced gas-oil ratio changes a little whenever the injection is miscible and increases rapidly in immiscible processes.