| Shengli oilfield contains many small scattered low-permeability reservoirs thatare usually deeply buried. These reservoirs are characteristic of strong diagenesis withdense cement and fine pores. Generally, the median value of radius is less than 1..52 m for most pores in the reservoirs. Therefore, connectivity of pores is extremelypoor, which is unfavorable to oil and gas flow in the reservoirs.In Shengli oilfield, waterflood is the principal method to exploit low-permeability oil reservoirs. However, it is difficult to conduct efficient water injection and oil production because of high sensitivity of the reservoirs which is resulted from high content of clay minerals in the main reservoirs. Common problems existed in the exploitation of low-permeability oil reservoirs includes under-injection of water-injection wells and injection-production imbalance. Consequently, it is desirable to develop new waterflood technologies in order to decrease injection pressure and increase production recovery for the low-permeability oil reservoirs.At present, waterflood of low-permeability oil reservoirs has become one of the major problems in oil and gas exploitation. Specifically, injection wells are incapable to efficiently absorb water, leading to vacant strata as well as difficulties in further injection of water. Meanwhile, current injection augmentation techniques are disadvantageous of losing validity in short period of time, regardless of its initial effectiveness. For example, acidizing, fracturing, and jet perforating treatments are only valid to improve injection for half a year in average and one year at most. Moreover, it is well known that single-well ground pumps are able to enhance injection pressure and water absorption of injection wells. Nevertheless, casing distortion may occur due to long-term high-pressure water injection by using the pumps. As a consequence, the injection-production technologies being currently employed can not satisfy the special requirements of exploiting low-permeability oil reservoirs. Widespread exploration of new technologies have been conducted in this area, however, no breakthrough was reported so far.Due to the absence of appropriate technologies to develop low-permeability oil reservoirs in Shengli oilfield, a petroleum sulfonate system has been invented to lower water-injection pressure and raise water-injection efficacy. By injecting the system into formation at a certain concentration, oil/water interfacial tension can be reduced along with change of oil emulsification and rock wettability as a result of interactions of surfactant in the system with reservoir formation and fluids. Emulsified oil possesses weak tendency of re-adsorption on formation surface during its movement in the reservoir. Also, strong electric repulsion exists between oil droplets and formation surface because of impact of anionic surfactant in the injected petroleum sulfonate system. Hence, oil droplets are easily carried away by flooding media, which elevates oil-wash capacity in the reservoirs. With more oil washed out from formation, oil belt emerges through collision of oil droplets. As oil belt moves forward, it is gradually getting bigger by combining scattered oils on the way, and finally can be produced from wellbore. In this situation, waterflood channel is significantly widened and waterflood resistance is brought down, giving rise to depressurization and efficiency enhancement of water injection.In this paper, we will introduce features of low-permeability oil reservoirs in Shengli oilfield as well as current status of water injection for the reservoirs. Besides, we will investigate water-injection augmentation mechanism of petroleum sulfonate, develop an injection augmentation system based on petroleum sulfonate, and evaluate performance and feasibility of the system in field applications.(1) Low-permeability reservoirs in Shengli oilfieldShengli oilfield contains abundant resources of petroleum deposits in low-permeability reservoirs. These reservoirs are widely distributed in 123 units that are presently developed by 9 oil-production plants. The major way to exploit these reservoirs is waterflood, due to the absence of other oil production technologies particularly for Low-permeability reservoirs. Nevertheless, oil and gas flow are negatively influenced by low connectivity of pores in the reservoirs. Furthermore, water injection and oil production are difficult to conduct because of high sensitivity of the reservoirs resulting from high content of clay minerals in the main reservoirs. Therefore, big challenges exist in the exploitation of low-permeability oil reservoirs, such as under-injection of water-injection wells and injection-production imbalance.(2) Water-injection augmentation mechanism of petroleum sulfonateAs an important attempt for waterflood technology, petroleum sulfonate has been blended with ahcohol, brine, and distillate oil (or crude oil) at a certain ratio to generate micelle, micro-emulsion, or oil solution depending on reservoir conditions. Then, the mixture is injected into oil reservoir with the dosage generally between 1%～20% based on the pore volume of the reservoir formation. Since the mixture is able to form a miscible phase with oil residual in the reservoir, oil/water interface is eliminated, which overcomes capillary effects and resulted difficulty of oil-residual capture. Thus, oil residual in the formation can be easily carried toward production wells, causing the decrease of core displacement pressure. The core simulation experiments have showed reduction of oil/water interfacial tension, oil-residual saturation, and water-injection pressure, as well as escalation of water-phase relative permeability and water-injection capacity, upon injection of the petroleum sulfonate system.(3) Property evaluation of the petroleum sulfonate productWater-injection improvement by petroleum sulfonate for low-permeability oil reservoirs are directly associated with the contents of surfactant, un-sulfonated oil, salt, and impurity in the system. In terms of water-injection augmentation mechanism of petroleum sulfonate, reduction of oil/water interfacial tension and promotion of oil-wash efficiency by surfactants in the system are the key factors to enhance water injection and oil production for low-permeability reservoirs.(4) Development and assessment of the injection augmentation systemAn injection augmentation system has been developed with Shengli petroleum sulfonate as the primary ingredient, based on the oil/water properties and formation features. Various lab experiments and studies of the system exhibit sharp reduction of interfacial tension (from original 10mN/m to the super low level of 10-3mN/m) between crude oil and formation water for Shi 3-10-inclined 12 production well. This phenomenon indicates dramatic improvement of water injection in low-permeability oil reservoirs. In this paper, we systematically evaluate properties of the petroleum sulfonate system, including oil/water interfacial-tension reduction and oil-wash efficiency enhancement, as well as adsorption loss of the system in formation cores. Additionally, we have studied the injection mode and dosage of the petroleum sulfonate system, and relationship between core permeability/plugging types and injection augmentation effects of the system.(5) Field application of the petroleum sulfonate systemXianhe Shishen 100 low-permeability oil reservoir was selected for the field trial of the petroleum sulfonate system. The water injection status for Shi 3-10-inclined well has been recorded upon injection of the petroleum sulfonate system. In comparison, status of the adjacent water-injection well was also recorded due to the same water resources for both Shi 3-10-inclined well and the adjacent water-injection well.Laboratory experiments and field trials have showed good performance of the petroleum sulfonate system in augmenting water injection for low-permeability oil reservoirs. A series of techniques have been established as to the injection boosting system, including formulation development, product evaluation, injection-parameter optimization, application design, and effect analysis.Implementation of the petroleum sulfonate system in Shi 3-10-inclined 12 well demonstrated reasonable water-injection pressure, start-up pressure, and water-absorption index. In contrast, the adjacent water-injection well exhibited rapid rising of injection pressure, and stable water injection could not be reached after multi-operations. This indicated that a remarkable improvement of water injection in low-permeability oil reservoir was achieved for Shi 3-10-inclined 12 single well by adopting the petroleum sulfonate-based injection augmentation technology. Therefore, success of the field trial would provide profitable information and build up solid foundation for the further large-scale application of the technology in low-permeability oil reservoirs.
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