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The focus on unconventional technology and operations, has been relentless during the last 15 - 20 years, while conventional operations have continued to tick along delivering their effective solutions globally. However, with unconventional operations dominating, it is inevitable that we run the risk of a fading knowledge base regarding the valuable contributions and hard lessons that have been learned with conventional techniques such as Tip Screen Out (TSO). This paper will present a global update on the development and application and continued success of this very specialised technique.The paper will describe the original development of the TSO process, design, deployment, refinement, and its broader application. A suite of case histories will demonstrate that every major operator in every major basin worldwide has successfully applied this technique to enhance production, where its use was both applicable and conditions made it possible. From highly specialised applications in North Sea chalks, to field developments in higher and medium permeability in Alaska and Siberia. From utilisation as an enabling solution in gas-condensates of South America, Middle East, and South-East Asia; and additionally, with the development of the Frac-Pack technique, delivery of a key sand-control completion method, crucial to GoM, Brazil and Global soft-rock oil production delivery.The paper will describe a range of requirements behind each consideration of deployment of the TSO technique, as well as specific in-situ characteristics that are required to support such application. It will describe the nuances of fracture design, material utilisation and adjustments that may be required to ensure effective delivery. The paper will also outline examples where the TSO process was the difference between success and failure. Finally, the paper will also cover some of the surveillance approaches utilised allowing a direct confirmation of the TSO process. All the extensive supporting evidence for this application will show how invaluable this technique has been to the Oil & Gas industry.In summary the paper will demonstrate the value which this technique has delivered in all its varied forms of application. It will enshrine the knowledge and lessons learned over 40 years of application and ensure that any short-term technical direction does not run the risk of disregarding the previously hard- won experiences of previous decades. Enshrined as an option in conventional fracturing techniques, the TSO process demonstrates the longevity that is associated with fundamentally sound engineering.

Group I intron ribozymes can repair mutated mRNAs by replacing the 3′-terminal portion of the mRNA with their own 3′-exon. This trans-splicing reaction has the potential to treat genetic disorders and to selectively kill cancer cells or virus-infected cells. However, these ribozymes have not yet been used in therapy, partially due to a low in vivo trans-splicing efficiency. Previous strategies to improve the trans-splicing efficiencies focused on designing and testing individual ribozyme constructs. Here we describe a method that selects the most efficient ribozymes from millions of ribozyme variants. This method uses an in vivo rescue assay where the mRNA of an inactivated antibiotic resistance gene is repaired by trans-splicing group I intron ribozymes. Bacterial cells that express efficient trans-splicing ribozymes are able to grow on medium containing the antibiotic chloramphenicol. We randomized a 5′-terminal sequence of the Tetrahymena thermophila group I intron and screened a library with 9 × 106 ribozyme variants for the best trans-splicing activity. The resulting ribozymes showed increased trans-splicing efficiency and help the design of efficient trans-splicing ribozymes for different sequence contexts. This in vivo selection method can now be used to optimize any sequence in trans-splicing ribozymes.

Group I introns are pre-mRNA introns that do not require the spliceosome for their removal. Instead, they fold into complex three-dimensional structures and catalyze two transesterification reactions, thereby excising themselves and joining the flanking exons. These catalytic RNAs (ribozymes) have been modified previously to work in trans, whereby the ribozymes can recognize a splice site on a substrate RNA and replace the 5′- or 3′-portion of the substrate. Here we describe a new variant of the group I intron ribozyme from Tetrahymena that recognizes two splice sites on a substrate RNA, removes the intron sequences between the splice sites, and joins the flanking exons, analogous to the action of the spliceosome. This 'group I spliceozyme' functions in vitro and in vivo, and it is able to mediate a growth phenotype in E. coli cells. The intron sequences of the target pre-mRNAs are constrained near the splice sites but can carry a wide range of sequences in their interior. Because the splice site recognition sequences can be adjusted to different splice sites, the spliceozyme may have the potential for wide applications as tool in research and therapy. © 2014 Amini et al.

With a global paradigm shift towards exploring shale reservoirs, the industry focus has moved towards optimizing hydraulic fracturing in these reservoir types. Low-viscosity slickwater fracture treatments are commonly used as a completion technique in these ultra-low permeability reservoirs. Each shale reservoir is different due to the presence of in situ natural fractures and other geologic complexities, and thus the resultant hydraulic fracture network is distinct and the proppant transport in these "created" complex fracture networks is not clearly understood. Much speculation exists in the industry as to how efficiently the proppant is transported from the primary fracture into subsidiary fractures, if it is at all. A better awareness of proppant movement in complex fracture networks can possibly help with better hydraulic fracture treatment designs by focusing on parameters that enhance transport in the subsidiary fractures and understanding what impacts this transport may have on the resulting production. This paper discusses a series of tests carried out in a low-pressure laboratory setting to evaluate proppant transport in complex fracture networks. Different slickwater treatment scenarios were simulated by pumping sand slurry through a series of complex slot configurations while varying the slot complexity, pump rate, proppant concentration, and proppant size. Results from twenty-seven tests carried out provide some interesting insights into the nature of proppant transport and settling in complex fracture networks. In the case of a primary slot system, the proppant transport was observed to occur via traction carpet after the creation of a proppant dune. However, in the case of secondary slots, the proppant transport was found to be dependent on the dune buildup in the primary slot. Two mechanisms were observed to be transporting the proppant into the secondary slots: 1) proppant flowing around the corner at pump rates higher than the threshold pump rate (related to the threshold velocity in the primary slot), and 2) proppant falling from the primary slot due to the effects of gravity, regardless of the pump rate.

How does a non-coding RNA evolve in cells? To address this question experimentally we evolved a trans-splicing variant of the group I intron ribozyme from Tetrahymena over 21 cycles of evolution in E.coli cells. Sequence variation was introduced during the evolution by mutagenic and recombinative PCR, and increasingly active ribozymes were selected by their repair of an mRNA mediating antibiotic resistance. The most efficient ribozyme contained four clustered mutations that were necessary and sufficient for maximum activity in cells. Surprisingly, these mutations did not increase the trans-splicing activity of the ribozyme. Instead, they appear to have recruited a cellular protein, the transcription termination factor Rho, and facilitated more efficient translation of the ribozyme's trans-splicing product. In addition, these mutations affected the expression of several other, unrelated genes. These results suggest that during RNA evolution in cells, four mutations can be sufficient to evolve new protein interactions, and four mutations in an RNA molecule can generate a large effect on gene regulation in the cell. © 2014 Olson et al.

Valhall, an offshore field in Norway, is a black oil reservoir producing mainly through multi-fractured (with proppant) horizontal wells.1 The bottom hole flowing pressure is well below the bubble point and hence the fracture conductivity is significantly constrained by multiphase non-Darcy pressure drop losses. Geertsma in 1974 was the first to propose a multiphase Beta (non-Darcy constant) correlation as a function of permeability, porosity (size) and saturation. Recent testing to measure the non-Darcy pressure drop for multiphase flow in proppant packs was successful in verifying a strong relationship to saturation. Three different types of 16/20 proppants with three different resins and one of 12/18 and 20/40 were tested (total of eleven samples) in a conductivity cell under field conditions (temperature of 195 deg F and 1,500-6,500 psi confining stress), for a proppant loading of 4 Lb/ft2 The proppant was placed in the cell using a 30 lb crosslink borate X-link fluid with 4 ppt (lbs/1000 gal gel) of encapsulated breaker for 24 hours to allow fluid break prior to conducting the flow tests. The Geertsma type equation was used to match the laboratory data. The equation constants (for permeability, porosity and saturation) for each proppant/resin type were derived. The non-Darcy correlations for each proppant were used in a numerical simulator to determine the pressure drop in the fracture and to quantify the effects on production rates. The paper highlights the importance of proppant size for optimizing fracture design for wells producing under multiphase flow.

Proppant fractures along the horizontal laterals in the Valhall Field have become a standard completion method for the last eight years with over 150 proppant fractures completed to date. The development of the flank regions of the Valhall Field began, with the first of 14 wells completions, in March 2003. Lateral lengths of up 2,000 meters will be drilled from two new platforms placed on the North and South edges of the field. The chalk formation in the flank regions is expected to be more competent then the crestal part of the field, so the question was raised as to whether fracturing should be done with acid or proppant. From a proppant fracturing perspective, each flank well will require between 10 to 14 prop fractures along its lateral requiring 2.5–3.7 Million pounds of proppant per well. Three different methods have been used to determine whether the wells should be acid or proppant fractured. These consist of reviewing the historical well performance, analytical and numerical modeling. All three methods clearly showed proppant fracturing was the preferred stimulation for the Valhall Field regardless of it's location. Acid fracturing becomes the stimulation of choice only if the well does not come in contact with enough OOIP (e.g. 5 MMSTB) to justify the proppant fractures. Proppant fracturing is expensive, so in conjunction with identifying the best stimulation method for the flank region, optimization with respect to fracture spacing along the horizontal lateral, fracture length and width have been numerically modeled for both the crestal and flank wells. This is an evolving process that should be considered an industry "Best Practice" as it enables real time optimization of ‘prop’ fracturing along a horizontal lateral during the drilling and completion phase. Since the start of this "Best Practice" in 2002, the wells stimulated in the crestal part of the Valhall Field have had the highest productivity in the field's history.