The LCUs' effect was not verified. Values of DC and KHN for LS decreased with increasing depth. The highest values for both DC and KHN were obtained at depths of 2-3 mm.

This paper was prepared for presentation at the 1998 SPE International Conference on Horizontal Well Technology held in Calgary, Alberta, Canada, 1-4 November 1998.

Freire,Federal Fluminense U.,and W. Campos, SPE, Petrobras C-M 199S, S.cu301y M Pebokwm Engtira, k This papa was prepared for proaentatii al the Fouth SPE Latn Amercan and Caribkan Pelmkum Er@nWmg GmWenca held In Pal c+Spah, Trin!dad & Tobe+ge22-26 A@l 1996 Thlo Papw WN sokdod W p+msan!mlbnby an SPE Program Ccmmiilea IOI!CIWIWrewew of nfanllcdm-1 MntakWd h m~lmd mbdled by lhn sulk.r[s). Ccmltis GI ha papIM how ml Mm rovicwad by W SO15DIYof PcW8u"n Engmem ati are 8UUOC410con'cdii by lfm -*).~m~~. aa PI=W*I dues IM necessarily rM6cJ any posilii of M Socisty of Psb'olmmnEngi-, its .affimm, or members Pqnm presenl~at SPE mealinps are $ubjecl @~* W Et-l~m~=s of~M~pe@OleumEWIIWS pWM~M to copy is rwbtctcd to a abdracl of rd mca than S02 words. Illuslratiis may nol be cepkd The abslmd SFWYJIII conlah umapiwcus @m.?wledpmenl of where and by wtcm he papar is =M& U~fim, SPE, P O Box SS2sSS, Rich-dscm, TX 7$Qs3sK%, U S.A. lax 01- AbstractThe study of stratified solid-liquid annular flow is relevant in several areas, e.g., mining and oil industries. A typical application is in the study of the removal of the rock formation cut by the bit during the drilling of a horizontal or highly inclined well. Seveml tbeoretieal models have been proposed for the description of this phenomenum. In these models, some empirical parameters are required, such as the shear stresses at the interface between the fluids and the cuttings bed and the maximum cuttings-wall tlktion factor which avoids bed movement. This work shows the"results of an experimental work developed on a 'large scale flow loop aiming at the quantification of these empirierd parameters. The tests consisted on the visualization of a saridstone bed erosion by different polymeric solutions flowing through an annular section. Pressure losses and steady state bed heights were recorded for several input parameters, such as fluid flow rate, rheology and density, annular geometry, eccentricity and particle size. SPE 36075

Cuttings transport has been the topic of theoretical and experimental studies for many investigators, who have always focused, due to the complexity of the subject, on steady state flows. However, the adequate representation of the involved phenomena requires more realistic simulation of the process, which is typically transient. The test results presented here relates to the erosion of a solids bed, formed in a full scale flow loop, by different polymeric suspensions in annular flow, at several values of flow rates, wellbore inclinations and drillpipe rotational speeds. Adequate instrumentation allowed for the recording of the evolution of solids concentration and bed height as function of time. Analysis of the experimental data enables the prediction of the rate and time of erosion of the cuttings bed and can be used as guidelines for the optimization of the circulation of horizontal and highly inclined wellbores. Introduction The action of stopping drilling to flush cuttings out of the well is always a point of controversy among oilfield teams. The practice of circulating periodically the hole may avoid several operational problems during the drilling of a highly inclined wellbore. On the other hand, if minimum requirements for the cuttings bed removal are not achieved, circulation will be ineffective, time consuming and, in some cases, detrimental to wellbore stability. The topic of drilled transport during drilling has been the subject of studies for many investigators (Iyoho, Sanchez, Peden et al.). who have all focused, on steady state flows. Martins et al. presented a discussion on the optimization of hydraulic parameters in the circulation of horizontal wells. In this study, the authors propose correlations for bed height prediction after circulation and for the critical flow rates for complete removal. The development of drilling activities requires more realistic simulation of the process, which is typically transient, due to interruptions in the operations and instabilities of rock formations, which tend to generate higher amounts of solids than expected. Ref .4, for example, does not answer to the major question about the optimum time to circulate a well. In the present work the authors propose a way to quantity the variation of the amount of solids removed from the annulus with time, during the circulation of horizontal and inclined wells. The main objective is to provide methods to evaluate the evolution of bed erosion for given hydraulic conditions. The study is based on an extensive experimental work conducted at University of Tulsa large scale physical simulator, as a cooperative program among PETROBRAS, SHELL INTL. E&P RTS and U. TULSA. Experimental Facility About 60 different hydraulic and geometric conditions were tested at the University of Tulsa (TUDRP) cuttings transport facility. The TUDRP flow loop has been designed and constructed by Stenevick and later modified by Bassal. It has been designed to simulate a wellbores in full scale and the main purpose is to perform research on cuttings transport experimentation. The flow loop is made up of a 84 ft long test loop section, composed of a 4 1/2 in drillpipe and an 8 in transparent acrylic outer pipe. Other components are a mud tank with an agitator of 85 bbls volume capacity, two parallel duplex mud pumps (one driven by a six cylinder diesel engine and another by an electric motor), flow lines connecting pumps via test loop section to mud tank, a cuttings injection tank and a cuttings collection tank (Fig 1).

Summary The problem of cuttings transport in the inclined sections of extended reach wells (ERW) is a major concern in the oil-well drilling industry. This article presents an innovative time-dependent mathematical model, along with a computer implementation of it, which is based on the well-known two-layer model approach. An innovative feature of this model consists in the consideration of the added amount of solids that comes from the crumbling and cave-in of the wellbore. The unstable effects of this added amount of solids is taken into account through a variable representing the added volume of cuttings per unit time and length. A finite volume approach is used to solve the system of differential equations. The paper represents a first step on the integrated analysis of cuttings transport and wellbore stability problems. Introduction The strategy of using ERW is being adopted worldwide as an attractive way of developing oilfields. Cuttings transport and wellbore stability are critical aspects in the drilling of highly inclined extended reach wells through unstable shales, which may be exposed to the drilling fluid during long time. These two problems are addressed by two classical sciences (two phase flow and rock mechanics) and treated separately. The study of steady-state stratified solid-liquid annular flows has been the subject of several researchers in the last decades1–3 in an attempt at describing the complex phenomena involved in the cuttings transport in high angle wells. On the other hand, considerable research effort has been conducted in the past aiming the quantification of mechanical4,5 and chemical6,7 effects which may cause wellbore instabilities. The main objectives of this paper are to introduce some ideas about the dependence of these two problems and to quantify the removal of solids generated by wellbore instabilities. The tool for this development is a computer program for the evaluation of cuttings circulation based on an innovative time dependent model for cuttings transport simulation. The computer program can calculate the variables which govern cuttings transport for the highly inclined section of an extended reach well, considering other source of solids besides the bit. Using the computer simulator, a series of analysis is performed in order to quantify the effects of the main variables as well as of the subtle increase of solids a given section of the well in the time-dependent behavior of cuttings transport. Cuttings Transport Simulation and the Two-Layer Model The two-layer model has been originally proposed for the representation of drilled cuttings transport in high angle wells,8 based on the good results of this approach for the simulation of other industrial processes, such as specific gas-liquid flows,9 and hydraulic transport of solids in pipes.10 The model proposed by the authors is illustrated in Fig. 1 and is based on the following hypothesis:The bottom layer represents the cuttings bed, which deposits in the annulus due to the action of gravitational forces. In this layer a fixed solids concentration of 52% is assumed.The top layer only contains the carrier fluid.There is no slip between the solid and liquid phases in each of the layers.There is no mass transfer between the solid and liquid phases.The solid-liquid system is incompressible and its rheological parameters are the same of the fluid.The height of the interface between the two layers is constant in the annular section studied and, consequently, a hydrostatic distribution of pressure in a cross section is assumed.The solids are characterized by an average diameter and by their sphericity. This model8 can be formulated by a system of two algebraic equations, based on momentum conservation laws for the bed and the fluid layers. The solution for the two unknowns, bed height and friction losses, require iterative approaches due to the nonlinearity of the system. Martins11 expanded the previous model, considering solids in the upper layer and four different flow patterns in the annulus: stationary bed, moving bed, heterogeneous suspension and pseudo homogeneous suspensions. The model is an adaptation of the proposal of Doron et al.12 for annular flows. This model is formulated by a system of four algebraic and one integral equations based on mass conservation laws for each phase, momentum conservation laws for each layer and a turbulent diffusion concept to describe solids distribution in the upper layer. This model has became the basis of PETROBRAS cuttings transport simulator which is widely used in the design and troubleshooting while drilling complex trajectory wells. Several improvements were made to the model, based on the feedback from field operators. Such improvements include specific correlations for interfacial stresses prediction,13 corrections for rotation effects,2 use of different rheological models for characterizing the fluid flow14 and the introduction of a permeability equation to describe flow through the bed.14 Among the changes in the basis model, proposed by Gavignet and Sobey,8 the introduction of a solid phase in the upper layer and the inclusion of an equation for liquid flow through the bed showed negligible difference in the final results. For this reason, these changes will not be kept in the time-dependent formulation. All the others proved to be very useful for the adequate representation of cuttings transport while drilling high angle wells. An expressive contribution was given by Nguyen and Rahman15 by the introduction of the three layer model concept to the analysis of cuttings transport. This concept, also brought from the hydrotransport industry16 may be useful to improve the understanding of the role of rheology on cuttings transport. The Time-Dependent Formulation The present work consists on a first step on time-dependent solid-liquid stratified flow modeling. The concept of the two-layer model is kept, but now, unlike any previous approach for cuttings transport analysis, transient conservation laws are derived to describe the phenomena. Eccentricity is an input parameter in the model. It directly affects all the area and perimeter expressions, as described in Gavignet and Sobey.8 Eccentricity can be predicted by mechanistic models17,18 using commercial softwares.