Hedi Sellami

PDC Bits: All Comes From the Cutter/Rock Interaction

PDC drill bit performances in hard rock has been greatly improved during the last decades by innovations in PDC wear, impact resistance and better vibrations understanding. The bit design is generally done by balancing the bit, distributing uniform wear along the profile and achieving high drillability and steerability. To obtain required drilling performances, drill bit designer adjust features such as profile shape, gage and mainly cutter characteristics (shape, type and orientation). Cutter rock interaction model became a critical feature in the design process. But previously used models considered only three forces on a cutter based on the cutter-rock contact area : drag force, normal force and side force. Such models are no longer valid with the introduction of PDC cutters with chamfer and special shape. This paper presents a new cutter rock interaction model including some several improvements. It is based on the presence of a build-up edge of crushed materials on the cutting face often described in the literature. In addition, the chamfer, which significantly affects bit Rate Of Penetration (ROP), is taken into account (shape and size). Forces applied on the back of the cutter and due to the rock deformation and back flow of crushed materials are considered in the model. Finally, results of numerous single cutter tests (under atmospheric and confining pressure) are presented and compared to the new cutter rock interaction model predictions. An analysis of the influence of the PDC characteristics (shape, size, chamfer, back and side rake angles, ....) is presented. The model has been applied to optimize the cutting efficiency and bit steerability and some design rules are given to minimize the specific energy and maximize the rate of penetration. Finally, full scale laboratory drilling tests and field results indicate that the use of accurate cutter rock interaction model can help the drill bit designer to find the best drill bit for a specific application. Standard laboratory full scale drilling procedures have been developed. The tests have shown that drillability, stability, steerability and wear can be improved and controlled by acting on the cutter characteristics, cutter setup, trimmer characteristics and gage type.

A Stick-slip Analysis Based on Rock/Bit Interaction: Theoretical and Experimental Contribution

The full-length paper analyzes bit stick/slip motion by coupling rock mechanics with field bottomhole data collected by use of an instrumented polycrystalline- diamond-compact (PDC) bit, a new tool that measures acceleration, weight on bit (WOB), and torque on bit (TOB) directly above the bit. Tests were conducted in Montrose, Scotland at approximately 700 m in a consolidated Devonian sandstone. The effect of stick/slip on rate of penetration (ROP) was quantified. Generally, an increase in WOB makes the system move toward stick/slip mode.

Buckling of Tubulars in Simulated Field Conditions

First, this paper presents the new developments integrated in a recently advanced model for drillstring mechanics and takes into account the buckling phenomenon in any actual well trajectory. Second, this paper shows the influence of tortuosity and friction on the buckling phenomenon for some practical and critical cases met in the drilling industry. These tortuosity and friction effects are demonstrated with an experimental setup that confirms theoretical features. Finally, we compare results obtained from existant models with results obtained from our new model to evaluate the tortuosity and friction effects on the critical buckling load found in the literature.

Axial Force Transfer of Buckled Drill Pipe in Deviated Wells

Axial force transfer is an issue in deviated wells where friction and buckling phenomenon take place. The general perception of the industry is that once drill pipe exceeds conventional buckling criteria, such as Paslay-Dawson, axial force cannot be transferred down-hole anymore. This paper shows that, even though buckling criteria are exceeded, axial force transfer could be still good if drill pipe is in rotation. On the contrary, there exists sliding operations where lockup is observed, due to buckling, even though standard buckling criteria are not exceeded. This paper is intended to show and explain how axial force is transferred down-hole in many simulated field conditions: sliding, rotating, with or without dog legs. These new results have been obtained from an advanced model dedicated to drill string mechanics successfully validated with laboratory tests. This paper will show applicable results for practical well operations where axial force transfer is an issue. These results will enable to give some guidelines to help the drilling engineer to select cases where conventional buckling criteria should be used cautiously. Indeed, simultaneous torque-drag-buckling calculations show that tubular can tolerate significant levels of compression, enabling to provide weight transfer to the drill bit, even though drill pipe is buckled. Others examples, in contrast, show that standard buckling criteria cannot predict the occurrence of buckling that may cause tubular lockup while tripping in the hole. The applications of these results are numerous for all deviated wells such as horizontal or extended reach drilling wells. This paper should contribute to reduce unpredictable lock-up situations and improve axial load transfer performance.

Analogous Reservoirs to Chicontepec, Alternatives of Exploitation for this Mexican Oil Field

The giant Chicontepec field contains oil from 18 to 45 o API in laminated sandstones of 0.1 to 10 mD at a depth of around 2500 meters (8202 ft). Original Oil in Place (OOIP) is estimated to be 140, 900 MMSTB. The complex geology (complicated structural and stratigraphic nature of the reservoirs), lack of reservoir information and lack of technology availability caused a gap between discovery and development. Throughout a period of several decades some exploration wells were drilled based on 2D seismic and log correlations of the reservoirs. The exploitation of the Paleonchannel was postponed because most of the wells showed poor productivity. The reasons for the low recovery (around 3%) have never been thoroughly understood. Various hypotheses have been proposed to explain the deficient performance such as partial closing of the fractures with declining reservoir pressure (bubble-point pressure is near initial pressure), inadequate comprehension of the geological model, deficiency in the fracturing technology, oil-wetted or intermediate-wetted reservoirs, applicability of unconventional wells (horizontal wells, casing drilling technology), etc. Today, the Chicontepec Paleochannel is an intermediate stage. Due to the experience of different fields with similar characteristics, this paper describes an analysis of alternatives that may be considered to resolve the problems of exploitation at the Chicontepec field. Advanced technologies, hydraulic fractures, artificial lift systems, all of them combined with secondary and enhanced oil recovery, may be feasible to sustain or increase production. A number of hurdles will have to be overcome. This field, the second most important oil field in Mexico, should take advantage of the experience learned from these analogous reservoirs. Chicontepec Paleochannel Geographically, it is located in east-central Mexico in parts of the states of Veracruz, Puebla and Hidalgo. Chincontepec system was deposited under complex tectono-stratigraphic conditions. Geologically, it covers an area of 957,534 acres (Figure 1). Aproximately half of Chicontepec consists of shales or silty shales with the rest of the formation made up of multiple thin sandstones beds and zones of sandstones beds. Typically, between 8 and 16 major reservoirs are present. These set of reservoirs is composed of channel complexes that are flanked by, and rest on, lobe sandstones that grade into distal fan and basin floor deposits, resulting in high heterogeneity. Throughout a period of several decades some exploration wells were drilled based on 2D seismic and log correlations of the reservoirs. The 3D seismic allowed the identification of sand bodies with viable pay thickness. Some wells produce small amounts of water, in general, water-oil contacts have not been identified. X-ray diffraction analysis showed that the clay cointains dominantly kaolinite with a content of 1 to 5 %. The sandstones are immature litharenites consisting of quartz grains, abundant carbonate fragments, and granitic fragments. Because of the abundance of carbonate in the system, the sediments are highly cemented by ferroan calcite and ferroan dolomite, in addition to quartz overgrowths. Core analyses show that the reservoirs are characterized by both low porosity and low permeability, Figure 2. All the reservoirs have permeabilities of 0.1 to 10 mD and porosities ranging from 5 to 15 %. The effective permeability, as determined from build up, fall off, drawdown and step rate test or advance decline analysis, varies from 0.01 to 15 mD.

How Drillstring Rotation Affects Critical Buckling Load

Buckling of tubulars inside wellbores has been the subject of many researches and articles in the past. However, these conservative theories have always followed the same assumptions : the wellbore has a perfect and unrealistic geometry (vertical, horizontal, deviated, curved), the friction and rotation effects are ignored, conditions relatively far from actual field conditions. How do tubulars buckle in actual field conditions, that is, in a naturally tortuous wellbore with friction and rotation ? Can we apply theories developed for perfect well conditions (no tortuosity, no friction, no rotation) to actual well conditions ? For the first time, this paper presents how the drillstring rotation affects the critical buckling load in actual field conditions. These new results have been obtained from an advanced model dedicated to drillstring mechanics successfully validated with laboratory tests. Firstly, this paper presents the new developments integrated in a recently advanced model for drillstring mechanics that enables to take into account the buckling phenomenon in any actual well trajectory. Indeed, some simultaneous torque-drag-buckling calculations are presented and allow to properly take into account the additional contact force generated in a post-buckling configuration, and as a consequence the additional torque at surface. Secondly, this paper shows the influence of friction and rotation on buckling loads for some practical and critical cases met in the drilling industry. These friction and rotation effects are demonstrated with an experimental set up that enables to confirm theoretical features. Lastly, this paper shows that using standard buckling criteria may lead to too conservative solutions, and that under specific circumstances, the drilling and completion engineer could safely operate in a buckling mode for a given time. These new results presented in this paper should improve significantly well planning and operational procedures to drill and operate more and more complex wells.