How to Improve Rate of Penetration in Field Operations

Abstract

M.J. Fear, SPE, BP Exploration Company (Colombia) Ltd. Abstract A method has been developed that identifies which factors are controlling rate of penetration (ROP) in a particular group of bit runs. The method uses foot-based mud logging data, geological information, and drill bit characteristics, to produce numerical correlations between ROP and applied drilling parameters or other attributes of drilling conditions. These correlations are then used to generate recommendations for maximising ROP in drilling operations. Introduction The time devoted to drilling ahead is usually a significant component of total well cost. On wells drilled by BP Exploration (BPX) in the last 5 years, this "rotating time" has accounted for 10-30% of the dryhole cost of the well. Because this rotating time is directly and inversely proportional to the ROP achieved by the drill bit, bit penetration rate has considerable significance in both control of drilling costs, and scope for cost reduction. Despite this, the interaction between the drill bit and rock, and the ROP that results; remains a part of the drilling process that is not particularly well understood in any level of detail. Other drilling phenomena, such as torque/drag behaviour, or the expected directional response of a bottomhole assembly, are typically managed with the aid of quantitative models, which have been validated against real well data and now offer a substantial improvement in understanding and reduction in uncertainty. Such tools are conspicuously absent from planning or analysis of bit performance. In the operational phase of drilling, similar uncertainty over expected ROP clouds decisions on which bit types to select, whether downhole motors or turbines could improve ROP, and which drilling parameters or bit nozzle arrangements would be most effective in maximising ROP. The contribution that intuition typically makes to these decisions testifies to the inability of drilling data or predictive models to provide anything more substantial. Against this background of commercial incentive and technical difficulty, BPX is involved in the development of methods which can be used to raise drill bit ROP. This paper describes one such method, and its application and benefits to a drilling operation. Background to the method Table 1 lists a number of factors which have been either proposed or observed to affect ROP. The number of factors is substantial, reflecting the complexity of the bit-rock interaction. This is compounded by observations that show significant inter-dependence between some of these variables, and non-linearity in some effects. Laboratory studies and modelling have however cast some light on how this complexity originates. First, how ROP responds to changes in drilling variables has been shown to depend strongly on the properties of the rock being drilled. In permeable rocks for example, overbalance pressure influences ROP, while this gives way to a dependance on bottomhole pressure as permeability decrease. Even overbalance pressure itself appears subject to dynamic influences as the bit advances, either via filtration effects on pore pressure in the zone being cut by the bit teeth, or stress effects on pore pressure around the wellbore. Bit cleaning problems while drilling hydratable formations in water based drilling fluids may also override the normal benefits of mechanical drilling parameters to ROP, so that rock mineralogy and drilling fluid chemistry are obviously significant "environmental" factors. These cleaning effects are though influenced by bit design and jet nozzle arrangement. In all, rock properties that influence bit response appear to include at least mineralogy, strength, density, porosity, and permeability. Given that these properties are not generally quantified while drilling in the field, it is not surprising that variations in ROP may be difficult to understand. Second, the effects of mechanical and hydraulic variables may also be intimately inter-dependant, so that the response of ROP to each of higher weight on bit (WOB), rotary speed and flow rate may vary with levels of the other parameters.

Elucidating PDC rock cutting behavior in dry and aqueous conditions using tribometry

“…The setting up and threading of the drill string during this exploratory stage can be costly, as well as dangerous at these extreme depths, especially given the unknown conditions and formations [1][2][3][4]. The largest costs incurred from drilling the final well are not only due to the energy spent drilling, but also from the replacement of worn bits, referred to as "bit trips", when the rate-of-penetration (ROP) becomes insufficient [1,[4][5][6][7][8][9][10][11][12]. After a bit failure of this kind, a project shut down must occur, and maintenance must be conducted that involves removal, disassembly, assembly, and rethreading of the drill string at the ocean floor.…”

“…The first is called the roller cone bit, in which three cone-shaped rollers are covered in cutters and allowed to freely rotate and crush the rock [11,[14][15][16]. While still used often in rotary-percussive drilling, this bit has largely been replaced in standard rotary drilling with the PDC bit, so named for its implementation of PDC cutters [4,5,8,9,12,14,17,18]. Since it features no moving parts, a large part of PDC bit design has been focused on the individual cutters, inspiring a greater drive to understand single-cutter rock interactions [9,19].…”

“…In addition to being a cause of energy dissipation, these are areas in which the contacting edge is actually flattened through excessive wear. After the loss in cutting edge, a much lower contact stress than that required for penetration and a subsequent reduction in ROP follows as a result [4,5,8,9,12,14,17,18,21].…”

“…Secondly, since the cut is kept at one constant depth, the data is generally far more uniform and dynamically simplified than cutters experience in the field [21]. This is due to the fact that the penetration into a rock surface during actual drilling will not remain constant, but will react to complex changes in the rock, normal load, or dynamics of the drill string [4,5,8,12,21]. This in turn has led to VTL studies being carried out primarily on extremely hard rocks such as white granite, to accelerate wear and make it more akin to that found in the field even on softer rocks.…”

“…13 and 14 present the data for COF and ROP at the same load, but for each rock substrate in dry and lubricated configurations respectively. While it is typical to present steady-state values for these types of rock comparisons[5,7,8], the transient data, as presented in Figs. 13 and 14, allows for more insight into how different the friction and ROP can be as steady-state is approached.…”