Challenges and Countermeasures Facing Casing Damage in Daqing Oil Field

Abstract

Abstract Casing damage is one of the most important problems influencing oilfield production. On the basis of the Daqing Oilfield characteristics, this paper analyzes the casing damage situation, shapes, horizon and characteristics of the damaged casing and discusses the harm of casing damage and the progress of casing damage mechanism. This article gives some suggestions for preventing and solving the casing damage. The result may be valuable to develop the similar oilfields. Introduction Casing damage is one of the most important problems for a water flooding oilfield in the whole world. The reservoirs of Daqing Oilfield belong to heterogeneous sandstone reservoirs, and they have been developed for 44 years at high production rate. By the end of 2004, casing damage wells amounted to 9,152 and the casing damage well percentage was 18.77, which seriously restrains oil and gas production and presents an austere challenge for the long-term and steady development of the oilfield. The main shapes of casing damage in the Daqing Oilfield are deformation, fault and fracture (Fig.1). Casing damage positions are concentrated in index bed and shale interlayer. The casing damage mechanism can be described as follows: the shale is soaked by water and forms water-soaked areas, which changes the character of the rock formation mechanics and the relationship among formation, interlayers. Water injection pressures are the direct impetus of formation slide and deformation; and creep, deformation and swell of shale is another reason for casing damage. Through numerical simulation, quantitative interpretation of casing damage is made. The measures have been put forward to prevent casing damage that establishes the basis for reducing and solving casing damage. 1 Casing Damage Situation Daqing Oilfield is one of the water injection oilfields in the world. The number of casing damage wells increases yearly. By the end of 2004, the total number amounted to 9,152 wells (Fig.2). It is 18.77 percent of the total number of injection and production well. It's not only seriously affecting the oilfield production but also resulting in huge economical lost. The annual economical lost is over $125 million. 2 Casing Damage Features The statistic data indicate that the casing damages appear seriously near the part of the structure axis and fault in the view of structure, and about 60% to 70% of casing damage locates in the layers of mudstone and shale and 30 to 40 percent in oil layers. The number of casing damaged wells varies with injection time and developing period. It depends on the production intensity. The greater the productivity is, the greater the casing damage is. Until now, seven blocks of serious casing damage wells are found. The casing damaged area is 2.2 sq. mile in the south of the northern Xing1–3B, and others range from 0.2 sq. mile to 1.1 sq. mile. In recent years, more and more wells have damaged in the oil layers. The number of casing damaged wells in oil zones is more than that in the part of non-oil zones. After many years' study, research shows the main reason for casing damage is that the water invades into mudstone and shale forming invading area. The anti-shear strength and inner friction coefficient of invaded mudstone and shale will decrease greatly. It will create different degree of creep deformation along with different water cut. Shale expands with creep deformation and squeezes the casing by certain outside force, so casing damage happens. Because of developed shale bedding structure, the resistance of shale is low after water invading and the invading area forms quickly. Due to the blocks pressure difference and the topography pressure difference produced by stratigraphic dip, the layer slides wholly and shears the casing to make casing damage. Non-equipoise reservoir pressure caused by separate zone water injection and non-equipoise water injection is the key condition of resulting in casing damage. In a water injection block, there must exist some high-pressure areas and some low-pressure areas because of non-equipoise reservoir pressure. In the high-pressure area, rock matrix expansion brings the around matrix to rise, in the low-pressure areas, rock matrix shrinkage leads to the surround matrix to decline. The rising and declining of the matrix make casing damage easily. Because some faults are unclear before drilling, the wells are improperly drilled near the faults. The pressure across the faults will increase along with water injection, which will result in pressure unbalance across the faults. When the unbalanced pressure difference rises greatly enough, the formations of faults can move, which leads to casing damage in the wells that go through the faults.