Most water-based reservoir drilling fluid systems used today are comprised of four primary components: the base brine, viscosifier, fluid-loss additive, and bridging particles. With the exception of minor adjustments of loading levels, the first three components normally do not change.The two primary types of bridging agents include calcium carbonate and sodium chloride salt. Some companies offer as many as seven different grades or sizes of each type of bridging particle. For the first time, the "ideal" pigment-size-distribution theory used widely in the paint industry has been transferred to practical oilfield use. This paper discusses the method and its use in selecting the optimum blend of bridging particles, focusing on an ideal packing sequence for minimizing fluid invasion. The authors examine the various procedures for optimizing sealing, as well as a management maintenance system. The paper expands on Abrams' Median Particle-Size Rule by going beyond the size of particle required to initiate a bridge. In the discussion, the authors examine the ideal packing sequence for formulating a minimally invading (non-damaging) fluid. Introduction Aside from minor adjustments of solids loading, the base brine, viscosifier and fluid-loss control additive in a traditional reservoir drilling fluid system change little from their original composition. However, the fourth primary component - the bridging particles - is used in a wide range of grades and sizes, depending on the anticipated extent of fluid invasion that must be thwarted. Thus, designing proper particle-size distribution is the first step towards formulating a minimally invading, non-damaging fluid. Historically, Abrams' rule1 has been used for this purpose. This rule states that "the median particle size of the bridging material should be equal to or slightly greater than 1/3 the median pore size of the formation." It goes on to suggest the concentration of the bridging solids must be at least 5% by volume of the solids in the fluid. In terms of particle size this means, for example, that 50µ particles should be effective at sealing pores up to or around 150µ in diameter. However, Abrams' rule only addresses the size of particle required to initiate a bridge. The rule does not give optimum size or address an ideal packing sequence for minimizing fluid invasion and optimizing sealing. While Abrams' rule has been the principle guideline for selecting particle size and concentration, others have used a "shotgun" approach to provide a broad and "all-encompassing" particle-size-distribution range. This approach assumes a single size distribution, often apparantly random, will cover the full range of pore throats or permeabilities that are likely to be encountered.2 However, this paper considers an ideal packing approach to the sealing problem, with the aim being to reduce formation damage and enhance the performance. This theory approaches reservoir drilling fluids from a reservoir-specific perspective. The theory works equally well for water-based (WBM) and oil-based (OBM) reservoir drilling fluids. Ideal Packing Theory - A Definition "Ideal packing" can be defined as the full range of particle-size distribution required to effectively seal all voids, including those created by bridging agents. This subsequent layering of bridging agents results in a tighter and less invading filter cake. The Ideal Packing Theory (IPT) introduced here takes a graphical approach to determine the optimum particle-size distribution of bridging material for given formation characteristics. The IPT uses either pore sizing from thin section analyses or permeability information, combined with particle-size distributions of the bridging material, to determine the Ideal Packing Sequence (IPS).

The efficacy of sarolaner (Simparica™, Zoetis) to prevent transmission primarily of Borrelia burgdorferi and secondarily of Anaplasma phagocytophilum from infected wild-caught Ixodes scapularis to dogs was evaluated in a placebo-controlled laboratory study. Twenty-four purpose-bred laboratory Beagles seronegative for B. burgdorferi and A. phagocytophilum antibodies were allocated randomly to one of three treatment groups: placebo administered orally on Days 0 and 7, or sarolaner at 2mg/kg administered orally on Day 0 (28 days prior to tick infestation) or on Day 7 (21 days prior to tick infestation). On Day 28, each dog was infested with approximately 25 female and 25 male wild caught adult I. scapularis that were determined to have prevalence of 57% for B. burgdorferi and 6.7% for A. phagocytophilum by PCR. In situ tick counts were conducted on Days 29 and 30. On Day 33, all ticks were counted and removed. Acaricidal efficacy was calculated based on the reduction of geometric mean live tick counts in the sarolaner-treated groups compared to the placebo-treated group for each tick count. Blood samples collected from each dog on Days 27, 49, 63, 77, 91 and 104 were tested for the presence of B. burgdorferi and A. phagocytophilum antibodies using the SNAP(®) 4Dx(®) Plus Test, and quantitatively assayed for B. burgdorferi antibodies using an ELISA test. Skin biopsies collected on Day 104 were tested for the presence of B. burgdorferi by bacterial culture and PCR. Geometric mean live tick counts for placebo-treated dogs were 14.8, 12.8, and 19.1 on Days 29, 30, and 33, respectively. The percent reductions in mean live tick counts at 1, 2, and 5 days after infestation were 86.3%, 100%, and 100% for the group treated with sarolaner 21 days prior to infestation, and 90.9%, 97.1%, and 100% for the group treated with sarolaner 28 days prior to infestation. Geometric mean live tick counts for both sarolaner-treated groups were significantly lower than those for the placebo group on all count days (P<0.0001). There were no adverse reactions to treatment with sarolaner. Transmission of B. burgdorferi to all eight placebo-treated dogs was confirmed by positive antibody (6 of 8 dogs), PCR (7 of 8 dogs), and/or culture (7 of 8 dogs). Similarly, transmission of A. phagocytophilum was confirmed by the presence of antibodies in four placebo-treated dogs. In contrast, treatment with a single dose of sarolaner prevented transmission of B. burgdorferi from infected ticks to dogs infested 21 or 28 days after treatment as demonstrated by negative antibody, PCR, and culture results. Prevention of transmission of A. phagocytophilum was demonstrated by negative antibody results in all sarolaner-treated dogs.

Quartz arenites are very clean quartz sandstones with unusually high permeabilities for any given porosity. This is attributed to the presence of large, clean pores which are often well connected. These large pores are susceptible to drilling damage, mainly from the invasion of solids. This paper presents both laboratory and field data on the development of drilling fluids to minimise damage in quartz arenites. The required changes in field mud engineering practices, to achieve improved well performance, are discussed. Field experiences are presented, including the impact of the modified mud systems and engineering practices on the resulting skin factors. Introduction Quartz arenites are very clean quartz sandstones with unusual poro-perm properties. Significant permeability is retained at low porosities and as the porosity increases the observed permeabilities are much higher than would normally be predicted. This unusually high permeability is attributed to the presence of large, clean pores which are often well connected. Despite these favourable characteristics, many wells drilled in quartz arenite reservoirs have not been as productive as expected, and have had high skin factors. In the absence of reactive clays, many recognised damage mechanisms are not applicable in quartz arenites, yet drilling damage is still seen to occur. Laboratory data shows that conventionally designed drilling fluids can cause high levels of damage, but that mud filtrates alone do not cause significant damage. The main damage mechanism is solids invasion into the large pores, and it is shown that changes in drilling fluid design can minimise this damage. These modifications to the drilling fluid require changes in field mud engineering practices, in order to achieve improved well performance. Field experiences are presented, including the impact of the modified mud systems and engineering practices on the resulting skin factors. The Mirador Formation Quartz arenites are clean quartz sandstones, which contain more than 95% quartz. The Mirador formation in the Cusiana field in Colombia is a quartz arenite, with a very low clay content. The typical mineralogy is 78% quartz grains, 14% quartz cement and 8% porosity. Despite low porosity, permeabilities are high. For example, 8% porosity corresponds to about 100mD and 10% porosity to about 4o0mD, although permeability is dramatically affected by grain size. The permeability of the very coarse grained sand is about 800mD at 10% porosity, whereas the permeability of the fine grained sand is about 90mD at the same porosity. The completed intervals have a wide range of permeability, sometimes from less than 10mD to more than 5000mD (the &gt;1000mD sands generally have low thickness). The Mirador has very low vertical permeability, compared to horizontal (Kv/Kh = 0.1 or less, dependent on formation permeability). Reservoir pressure is approximately 5300psi and reservoir temperature is 127 F. Pore Size Distribution. The high permeability at low porosities is a result of large pore size and good interconnectivity. The median (D50) pore size in the lower Mirador is usually about 60, although in the high permeability streaks the D50 pore size has been measured as high as 110. More importantly, typical pore size distributions show the D95 to be substantially higher than the Dso (Fig. 1). As an example, a core with Dso pore size of 60 will have a D95 of 350-400. By converting a typical pore size distribution to a predicted permeability distribution (Fig. 2) it is apparent that the large pores dominate the permeability. In the example presented, the largest 30% of the pores account for about 85% of the permeability. Consequently, the large pores need to be protected in order to minimise damage. Compressive Strength. As a result of the secondary quartz cementation, which produced the low porosities, the Mirador sands also have high unconfined compressive strengths. P. 147

BackgroundWhile Koch’s postulates have been fulfilled for Lyme disease; causing transient fever, anorexia and arthritis in young dogs; treatment of sero-positive dogs, especially asymptomatic animals, remains a topic of debate. To complicate this matter the currently recommended antibiotic treatments of Lyme Disease in dogs caused by Borrelia burgdorferi require daily oral administrations for 31 days or longer, which makes non-compliance a concern. Additionally, there is no approved veterinary antimicrobial for the treatment of Lyme Disease in dogs in the USA and few recommended treatments have been robustly tested.In vitro testing of cefovecin, a novel extended-spectrum cephalosporin, demonstrated inhibition of spirochete growth. A small pilot study in dogs indicated that two cefovecin injections two weeks apart would be as efficacious against B. burgdorferi sensu stricto as the recommended treatments using doxycycline or amoxicillin daily for 31 days.This hypothesis was tested in 17–18 week old Beagle dogs, experimentally infected with B. burgdorferi sensu stricto, using wild caught ticks, 75 days prior to antimicrobial administration.ResultsClinical observations for lameness were performed daily but were inconclusive as this characteristic sign of Lyme Disease rarely develops in the standard laboratory models of experimentally induced infection. However, each antibiotic tested was efficacious against B. burgdorferi as measured by a rapid elimination of spirochetes from the skin and reduced levels of circulating antibodies to B. burgdorferi. In addition, significantly less cefovecin treated animals had Lyme Disease associated histopathological changes compared to untreated dogs.ConclusionsConvenia was efficacious against B. burgdorferi sensu stricto infection in dogs as determined by serological testing, PCR and histopathology results. Convenia provides an additional and effective treatment option for Lyme Disease in dogs.

Owing to the economic, well control and drilling efficiency problems it propagates, preventing or curing lost circulation has evolved into a critical issue for the industry. Accordingly, a less expensive drilling fluid system often is selected for a particular interval because it will mitigate the economic ramifications of downhole losses. Conversely, fluid systems are also chosen solely for their capacity to minimize losses downhole. Regardless of the application, special analyses are required to not only determine specific preventative measures for drilling fluid loss, but also for devising an appropriate treatment program to cure losses should they occur. Recently, specialized and chemically activated cross-linked pills (CACP) designed to stop whole mud loss have been developed and have shown significant advantages over conventional loss circulation methods. This paper describes the development and application of the specialized cross-linked pills for controlling downhole mud loss. The authors will detail the laboratory methods used to formulated appropriate formulations, the placement procedures, and the successful field applications of this latest generation lost circulation control mechanism. As detailed in the paper, formulations of the new cross-linked pills normally are tailored to specific downhole conditions of temperature and pressure to create a barrier between the loss zone and the wellbore. Further, they have been specially formulated to enable pumping through narrow drill pipe, coil, or mud motor dimensions and to "set" in a specific time frame. Chemical activators or retarders to control setting times achieve this. These pills also contain controlled concentrations of particulate sealing agents that can be used selectively to either to bridge the loss zone at the formation face or allow deeper penetration into the fracture. Introduction The problem of lost circulation was apparent in the early history of the drilling industry and was magnified considerably when drilling deeper and/or depleted formations1. The industry spends millions of dollars a year to combat lost circulation and the detrimental effects it propagates, such as lost of rig time, stuck pipe, blow-outs and, frequently, the abandonment of expensive wells. Moreover, lost circulation has even been blamed for minimized production in that losses resulted in failure to secure production tests and samples, while the plugging of production zones have led to decreased productivity2. The strata considered in this paper as so-called "thief" formations or loss zones include cavernous-type intervals, others containing fissures, cracks and crevices as well as porous, highly permeable sandstones and gravel beds2. Many products and techniques have been used to attempt to restore circulation while drilling. These include fibrous, flaky and granular materials2, as well as techniques such as gunk and reverse gunk squeezes, high fluid loss squeezes and cement squeezes. Development of a New Lost Circulation Material The theories and observations raised from years of experience fighting lost circulation can be boiled down to five pertinent points3,4:A lost circulation material should be equally effective in sealing unconsolidated formations and fractures or vugs in hard formations.It should form an effective seal under both low and high differential pressure conditions.Final plug shear strength should be high enough to support fluid column, but low enough to ensure removal by washing or jetting (low sidetrack risk).The plugging seal has to withstand both negative (swab) and positive (surge) pressures applied during drilling, drill pipe trips and casing runs.It should have workable/controllable set time and should be functional in oil-, synthetic- or water-based systems.