Effect of Prebent Deflection on Lateral Vibration of Stabilized Drill Collars

Summary An effective modulation of lateral vibration of a bottomhole assembly (BHA) is that some prebent deflection is made on the lower stabilized section of drill collars. This is because the restorable deflection of a drill collar confined in a borehole depends on the prebent deflection. If the prebend nears the limit that the borehole can contain, there will be no sufficient restorable deflection to produce restoring force for lateral vibration. To understand the nature of this phenomenon, a dynamic model for lateral vibration of a stabilized prebent drill-collar section in an inclined well is presented. The governing nonlinear equations are obtained by using a Lagrangian approach, which are characterized by a prebend-dependent nonlinear bending stiffness term. They also include the effects of clearance/friction of stabilizers with borehole wall, contact friction of drill collar with borehole wall, axial forces from weight-on-bit (WOB) and bit/formation interactions, and hydrodynamic damping of drilling mud. The gyroscopic moments are proved to be negligibly small terms within a range of operating conditions of drilling. Poincaré map bifurcation diagram, phase diagram, Liapunov exponent, and power spectrum analysis are used to evaluate the dynamic behaviors. Simulation results both in a vertical well and in an inclined well show that lateral vibration is sensitive to the prebent deflection in some degree, presenting periodic motion, chaotic motion, or alternating in turn as usual. However, when prebent deflection is more than 87% in a vertical well and more than 45% in a 20°-inclined well (of the average clearance between the drill collar and the borehole), lateral vibration will vanish. Such a characteristic is crucially important for practical applications. For example, we may effectively prevent a drill string in an inclined well from severe whirling motion with a properly designed prebent drill collar.</jats:p

Nuclear Magnetic Resonance Evaluation Study for Effect of Foam flooding in Heterogeneous Cores

Foam flooding demonstrated the ability to solve the viscous fingering problem of gas flooding and increase the sweep efficiency in enhancing oil recovery. It is commonly used in development of heterogeneous reservoirs. While the characteristics of fluid migration in pores and between layers were still unclear. In this paper, Dynamic change of oil and water with different foam quality was tested during foam flooding by NMR method. Oil displacement effect of water flooding and foam flooding was compared. The results showed the foam quality affected the foam stability and profile control effect. Compared with water flooding, the foam could increase the recovery rate of the low-permeability layer, and the foam system with high stability had a high sweep efficiency and a high oil displacement efficiency in the heterogeneous cores

Nuclear Magnetic Resonance Evaluation Study for Effect of Foam flooding in Heterogeneous Cores

Foam flooding demonstrated the ability to solve the viscous fingering problem of gas flooding and increase the sweep efficiency in enhancing oil recovery. It is commonly used in development of heterogeneous reservoirs. While the characteristics of fluid migration in pores and between layers were still unclear. In this paper, Dynamic change of oil and water with different foam quality was tested during foam flooding by NMR method. Oil displacement effect of water flooding and foam flooding was compared. The results showed the foam quality affected the foam stability and profile control effect. Compared with water flooding, the foam could increase the recovery rate of the low-permeability layer, and the foam system with high stability had a high sweep efficiency and a high oil displacement efficiency in the heterogeneous cores.</jats:p

Stability Analysis of Pipe With Connectors in Horizontal Wells

Summary Except for coiled tubing, most tubular goods used for downhole operations (such as drillpipe and sucker rod) have connectors. Because a connector and the pipe body have different outer radii, the deformation and buckling behavior of a pipe with connectors constrained in a wellbore is much more complicated. However, most buckling models were established by neglecting the existence and effects of connectors. In this paper, buckling equations of a pipe with connectors in horizontal wells were derived with application of elastic-beam theory. The axis of an unbuckled pipe is a 2D curve in the vertical plane and has three configurations—no contact, point contact, and wrap contact. We derived the two critical distances between connectors, Lc1 and Lc2, beyond which a pipe changes its configuration from one to another. The authors proposed an algorithm to determine the critical force (Fcrs) of buckling by numerically solving the buckling equations using the fourth-order Ronge-Kuta method. Both the distance between two adjacent connectors (Lc) and the radius difference between a connector and the pipe body (∆rc) have significant impact on the critical force, in addition to net clearance between a pipe and wellbore (r0), bending stiffness (EI), and weight per unit length (w) of pipe. When Lc is small, radial deflection is negligible. Fcrs increases as ∆rc increases. However, when Lc is close to Lc1, effects of radial displacement become significant, and Fcrs decreases dramatically as ∆rc increases. Fcrs decreases as Lc increases when Lc Lc1, and it reaches its minimum at Lc=Lc1. When Lc &amp;gt; Lc1, Fcrs fluctuates as Lc increases. Some curves of Lc1, Lc2, and Fcrs, all in dimensionless forms, were calculated and presented in this paper for practical applications. Our numerical results show that the critical force may reduce by 20 to 60% for commonly used drillpipes and sucker rods with centralizers, which indicates that a pipe string designed without considering the effects of connectors may be risky. The results presented in this paper may provide some practical guidance for optimal design of centralizers for sucker-rod strings, or may avoid some risks because of improper design of drillpipe strings.</jats:p