Routine Swabs Research Articles

Investigation using whole genome sequencing of a prolonged restaurant outbreak of Salmonella Typhimurium linked to the building drainage system, England, February 2015 to March 2016.

Following notification of a Salmonella enterica serovar Typhimurium gastroenteritis outbreak, we identified 82 cases linked to a restaurant with symptom onset from 12 February 2015 to 8 March 2016. Seventy-two cases had an isolate matching the nationally unique whole genome sequencing profile (single nucleotide polymorphism (SNP) address: 1.1.1.124.395.395). Interviews established exposure to the restaurant and subsequent case–control analysis identified an association with eating carvery buffet food (adjusted odds ratios (AOR): 20.9; 95% confidence interval (CI): 2.2 – ∞). Environmental inspections, food/water testing, and a food trace-back investigation were inconclusive. Repeated cycles of cleaning were undertaken, including hydrogen peroxide fogging, however, transmission continued. After 7 months of investigation, environmental swabbing identified 106 isolates from kitchen surfaces and restaurant drains matching the outbreak profile. We found structural faults with the drainage system and hypothesised that a reservoir of bacteria in drain biofilm and underfloor flooded areas may have sustained this outbreak. Ineffective drain water-traps (U-bends) may have also contributed by allowing transmission of contaminated aerosols into the kitchen environment. These findings suggest that routine swabbing of sink drain points and inspection of drainage systems should be considered in future outbreak scenarios.

Recurrent Acute Tonsillitis, The 'Core' Issue

Introduction:
 Today bacteriological and patho-anatomical considerations too are taken into account while treating a case of tonsillitis. Past decade has seen the rise of resistance amongst the common pathogens, as well as rise in the number of unusual offenders. Determination of the true offending organism and prescribing an antibiotic as per the sensitivity pattern is of utmost importance.
 Materials and Method:
 A prospective longitudinal study was conducted in a tertiary care hospital in Kolkata. The study population consisted of patients presenting with recurrent attacks of acute tonsillitis. Determination of throat swab micro flora, ASO titre and core tissue microflora was done and correlated statistically.
 Result:
 There was poor correlation between throat swab and core tissue microflora. Positive predictive value of throat swab was 10%. Pseudomonas is the predominant flora harbouring tonsillar core in our study population. Amoxicillin the most commonly prescribed antibiotic stands out to be the most resistant one. No statistical significance could be reached comparing streptococcal tonsillitis with ASO titre.
 Discussion:
 The real pathology within the tonsil core is not always reflected in routine throat swab and bacteriology of recurrent tonsillitis may differ in different regions. ASO titre estimation adds up to the economic burden if rheumatic fever is not suspected. Judicious use of antibiotics based on true sensitivity pattern is encouraged. In the era of antibiotics Tonsillectomy still holds an important position in the management of recurrent attacks.
 Conclusion:
 Pseudomonas was found to be the most common pathogen in recurrent acute tonsillitis. Core tissue study remains the Gold standard in identifying the pathogenic organism. Oral third generation Cephalosporin was the most efficacious antibiotic for recurrent tonsillitis in our study population.

Bacteriological cultures on admission of the burn patient: To do or not to do, that's the question

IntroductionIn many burn centers, routine bacteriological swabs are taken from the nose, throat, perineum, and the burn wound on admission, to check for the presence of microorganisms that require specific measures in terms of isolation or initial treatment. According to the Dutch policy of “search and destroy,” for example, patients infected by multiresistant bacteria have to be strictly isolated, and patients colonized with β-hemolytic Streptococcus pyogenes must receive antibiotic therapy to prevent failed primary closure or loss of skin grafts. In this respect, the role of bacteria cultured on admission in later infectious complications is investigated. The aim of this study is to assess systematic initial bacteriological surveillance, based on an extensive Dutch data collection. Materials and methodsA total of 3271 patients primarily admitted to the Rotterdam Burn Centre between January 1987 and August 2010 with complete bacteriological swabs from nose, throat, perineum, and the burn wounds were included. For this study, microbiological surveillance was aimed at identifying resistant microorganisms such as methicillin-resistant Staphylococcus aureus (MRSA), multiresistant Acinetobacter, and multiresistant Pseudomonas, as well as Lancefield A β-hemolytic streptococci (HSA), in any surveillance culture.The cultures were labeled as “normal flora or non-suspicious” in the case of no growth or a typical low level of bacterial colonization in the nose, throat, and perineum and no overgrowth of one type of microorganism.Further, the blood cultures of 195 patients (6.0%) who became septic in a later phase were compared with cultures taken on admission to identify the role of the initially present microorganisms. Statistical analysis was performed using SPSS 20.0. ResultsAlmost 61% of the wound cultures are “non-suspicious” on admission. MRSA was cultured in 0.4% (14/3271) on admission; 12 out of these 14 patients (85.7%) were repatriated. Overall, 9.3% (12/129) of the repatriated patients were colonized with MRSA. Multiresistant Acinetobacter or Pseudomonas was detected in 0.3% (11/3271 and 10/3271, respectively). In total, 18 of the 129 repatriated patients (14%) had one or more resistant bacteria in cultures taken within the first 24h after admission in our burn center.On admission, S. pyogenes was found in 3.6% of patients (117/3271), predominantly in children up to 10 years of age (81/1065=7.6%). ConclusionsResistant bacteria or microorganisms that impede wound healing and cause major infections are found only in few bacteriological specimens obtained on admission of patients with burn wounds. However, the consequences in terms of isolation and therapy are of great importance, justifying the rationale of a systematic bacteriological surveillance on admission.Patients who have been hospitalized for several days in a hospital abroad and are repatriated show more colonization at admission in our burn center. The microorganisms identified are not only (multi)resistant bacteria, showing that a hospital environment can quickly become a source of contamination. These patients should receive special attention for resistant bacteria. HSA contamination is observed more frequently in younger children. Bacteria present at admission do not seem to play a predominant role in predicting later sepsis.

69 The Cystic Fibrosis Sputum Induction Trial (CF-SpIT). Induced sputum in young healthy non-productive children with cystic fibrosis

Objective Young children with CF are often non-productive of sputum even during a chest exacerbation. Effective bacterial surveillance is challenging, but critical as respiratory infection and lung disease begins early in life. CF-SpIT (CF-Sputum Induction Trial: UKCRN 14615) seeks to identify whether using induced sputum (IS) improves bacterial surveillance, in young non-productive children with CF. Methods 86 children from CF-SpIT are included in this analysis. In all cases, a routine cough swab (CS) was followed by IS. Paired samples were compared for microbiology yield. In 20 children, where bronchoscopy was clinically indicated, CS, IS and extensive 6 lobe bronchoalveolar lavage (BAL) were compared for microbiology yield. Results IS was tolerated in 98% of patients, and successful in 81% (n = 69). Tolerability and success did not differ in children below or above 6 years of age, despite more of the younger cohort being naive to hypertonic saline (30%), not spontaneously productive (92%) and unwell at the time (68%). Of the 69 patients, 17% had positive cough swabs, 47% had positive IS samples (χ 2 = 10. P=0.002). IS identified a pathogen not previously isolated in the previous year in 29% of patients. In 20 patients where CS, IS and BAL were taken, 20 pathogens were grown in 16 patients. CS, IS and BAL identified 5, 13 and 14 pathogens respectively. Pathogens were identified on IS but not on BAL in 4 patients. 16S sequencing of culture-negative BAL was positive for the pathogen identified on IS in 3 of 4 cases. Conclusion Induced Sputum is a well tolerated, non-invasive, effective way to sample the lower airway in young non-productive healthy children with CF.

PTU-209 Intra-operative swab cultures do not predict the development of anal fistula in patients undergoing emergency incision and drainage of perianal abscess: Abstract PTU-209 Table 1

Introduction Routine swab cultures at the time of perianal abscess drainage remains common practice in modern day emergency colorectal surgery. We aimed to analyse the effectiveness of culture swabs to predict short term and long term outcomes of perianal abscess drainage. Method 350 consecutive patients from January 2008 to December 2010 undergoing incision and drainage (I&D) of perianal abscesses with packing of wound in community were analysed for readmission, follow up outpatient visit, repeat procedure for ongoing symptoms and presence of anal fistula at follow up examination under anaesthesia (EUA). Patients with inflammatory bowel disease and known anal fistula disease were excluded. Inter-group comparative analyses were made for patients with and without intra-operative culture swabs and for the microbial growth. Results Overall fistula formation rate after I and D was 12% at 36 month follow up. No statistical significant differences were found between the different groups for rate of readmission, presence of fistula and number of outpatient consultations (P > 0.05). There was no significant difference in fistula rates for patients with positive microbial growth and negative microbial growth (10% vs. 8%, p = 0.12). No significant variation was observed for different types of organisms and rate of fistula formation. (See Table 1). Conclusion Routine swab cultures do not influence short and long term outcome associated with I&D of perianal abscess. Disclosure of interest None Declared.

Blind vaginal fetal fibronectin swab for prediction of preterm birth.

To assess the accuracy of blind vaginal swab in diagnosis of preterm labor (PTL) and prediction of subsequent occurrence of preterm birth (PTB). Eligible women who presented at 24-34 weeks of gestation with threatened PTL had their cervicovaginal secretions sequentially sampled for fetal fibronectin (fFN) using two types of swabbing techniques. The first swab was a blind vaginal swab collected without the aid of speculum, while the second one was a routine cervicovaginal swab. All participants were followed up until delivery. fFN in both swabs was significantly higher in women who delivered before term compared with women who delivered at term. On regression analysis, r(2) = 0.735 and 0.785 for blind vaginal and cervicovaginal swabs respectively, while on receiver operating characteristic analysis the area under curve was 0.965 and 0.977, respectively, without a statistically significant difference. Using an fFN cut-off of ≥0.05 μg/dL, the sensitivity, specificity, positive predictive value and negative predictive value of blind vaginal swab to predict PTB were 53.9%, 91.9%, 70.0% and 85.0%, respectively; while those for cervicovaginal swab were 58.3%, 94.7%, 77.8% and 87.8%, respectively. In women presenting at 24-34 weeks of gestation with threatened PTL, blind vaginal fFN swab is as effective as cervicovaginal swab to predict PTB, but it is easier to perform and does not require doctor supervision.

Routine operative swab cultures and post-operative antibiotic use for uncomplicated perianal abscesses are unnecessary.

Routine swab cultures for perianal abscesses remain commonplace in surgical practice. However, patients are often discharged post-operatively prior to the culture results being made available. Consequently, intra-operative swab cultures rarely impact subsequent management and outcomes. Similarly, the use of broad-spectrum antibiotics for perianal abscesses post-drainage also remains prevalent, albeit with questionable benefit. The records of all patients diagnosed with perianal abscess from January 2011 to December 2011 were reviewed. Patients with complicated perianal abscesses or recurrent abscesses previously treated before the study period were excluded. The demographics, medical co-morbidities, intra-operative findings, swab cultures, microbiological results and use of post-operative antibiotics were reviewed. Subsequent wound healing and follow-up were also recorded. Two hundred and seven patients were admitted to our institution for perianal abscesses in 2011. After excluding 35, the remaining 172 patients were analysed. One hundred and thirty-four patients (78%) had swab cultures performed intra-operatively but 80% of these were discharged prior to the culture results being available. One hundred and eight (63%) were discharged with outpatient antibiotics. During the index admission and subsequent follow-up, swab culture results were not documented to be reviewed by the attending physician 96.5% of the time. Sixteen patients required repeat surgery for recurrence of anorectal sepsis. We found that the use of antibiotics after the index surgery did not confer a statistically significant benefit. Routine swab cultures are unnecessary and do not affect management and outcome. The use of post-operative antibiotics may reduce the rates of recurrence, but this benefit was not found to be statistically significant.

Mystifying lesions in syphilis-HIV co-infection

Case report We report the case of a 22 years old male patient, who presented with a vesicular eruption, bacterial over infected, with a group pattern distribution throughout the entire skin, evolving for approximately three months. The patient was previously treated for this condition with systemic antibiotics, which he could not mention, being diagnosed with superficial folliculitis. However, no improvement was noticed. He denied unprotected sexual intercourse. The patient was clinically examined and we performed routine tests, immunologic assays, bacteriological swab and skin biopsy. General examination showed generalized, painless micropoly-lymphadenopathy and continuous muscle pain in the lower legs. Dermatological assessment of lesions revealed a disseminated eruption on the entire skin, including the face and penis sheath, consisting of erythematous plaques with the surface covered by crusts and vesicles arranged in groups, with various sizes (diameter 13 cm) and clearly defined edges. The presumptive diagnosis was secondary syphilis, and in order to rule out the differential diagnosis of pityriasis lichenoides et varioliformis acuta, eosinophilic folliculitis and systemic lupus erythematosus we performed a series of tests including immunologic assays, viral markers for HCV, serology for syphilis, bacteriological swab from the lesions and a skin biopsy. The results were positive for syphilis and Staphylococcus aureus was isolated from the lesions. Histopathological appearance of psoriasiform and lichenoid dermatitis with plasma cells was interpreted in the context of secondary syphilis, confirming our diagnosis. The patient eventually admitted having multiple unprotected sexual contacts. We tested for HIV Ag/Ac (1 + 2), which was positive.

Human Papillomavirus Infection in Benign Tonsil Swabs and Tonsillectomy Specimens

Objectives: (1) Determine incidence of HPV (human papillomavirus) infection in routine tonsillectomy swabs and specimens. (2) Determine if vaccination impacts tonsillar HPV infection. Methods: This was a prospective study conducted from July 2012 to January 2014. Patients ages 1 to 100 years undergoing tonsillectomy for nonmalignant indications were enrolled. Demographic data and immunization status were collected. Specimens collected during surgery included a flocked swab and half of the left and right tonsil. All specimens were tested for HPV DNA using the polymerase chain reaction–based Roche Cobas Amplicor test and for the presence of E6/E7 mRNA using the Gen-Probe Aptima HPV test. Results: Patients were stratified into 2 groups, pediatric (ages 1-12 years) and adults (age 13 years and up) based on age of potential sexual maturity. In the pediatric group (n = 130) mean age was 5.2 years, 42% were female, 58% were male, and only 3 patients were fully vaccinated against HPV. In the adult group (n = 64) mean age was 24 years, 76% female, 24% male, and 47% were either fully or partially vaccinated. All specimens were negative for HPV infection via both testing methods. Internal controls confirmed the tests were functioning properly. Conclusions: This is the first study to examine fresh tonsil swabs and tonsillectomy specimens for HPV infection using a Food and Drug Administration–approved method. Interestingly, both adult and pediatric specimens were negative for HPV infection. The role of vaccination in oropharyngeal HPV infection remains unclear.

Use of Xpert® MRSA PCR point-of-care testing beyond the laboratory

A meticillin-resistant Staphylococcus aureus (MRSA) polymerase chain reaction assay (Xpert(®) MRSA, Cepheid) was assessed for point-of-care testing (POCT) used by healthcare assistants in an orthopaedic pre-admission clinic and on a vascular ward to reduce turnaround time. POCT results were compared with the routine swabs taken for culture. The POCT assay was easy to use, the turnaround time for negative results was greatly reduced, and sensitivity was 75.0% in the pre-admission clinic and 80.0% in the vascular ward. There were 27 POCT-positive/culture-negative results, but there was no evidence of MRSA infection or colonization in these patients for at least a year post procedure. POCT for MRSA colonization performed beyond the laboratory has important advantages over laboratory-based methods and should be explored further.

The Presence and Impact of Biofilm-Producing Staphylococci in Atopic Dermatitis

Atopic dermatitis (AD) is thought to be a double-hit phenomenon with an unknown environmental component and a genetic abnormality likely centered on the filaggrin gene. Biologically, the presence of Staphylococcus aureus in AD was reported more than 2 decades ago, but the relationship to AD has been elusive. To explore the bacteria that produce the biofilms in the lesions of AD and the response of the innate immune system to these biofilm occlusions of the sweat ducts by specifically evaluating Toll-like receptor 2. University hospital dermatologic clinic study involving the environmental component related to the characterization, correlation, and impact of staphylococci and their biofilms in AD. We processed routine skin swabs from lesional and nonlesional skin from 40 patients with AD and performed scrapings and biopsies. We also obtained 20 samples from controls (10 inflamed skin samples and 10 normal skin samples). Gram staining, bright-field microscopy, hematoxylin and eosin, periodic acid-Schiff, Congo red, and light microscopy. Association of staphylococcal biofilms with AD pathogenesis. All AD-affected samples contained multidrug-resistant staphylococci, with S aureus (42.0%) and Staphylococcus epidermidis (20.0%) as the predominant species. All isolates were positive for extracellular polysaccharide and biofilm (85.0% strong biofilm producers and 15.0% moderately to weakly positive). Polymerase chain reaction revealed the biofilm-mediating icaD (93.0%) and aap (12.5%) genes in the isolates (some contained both). We also examined tissues for microbial identification, extracellular biomass formation, biofilm formation, and staphylococcal biofilm in skin tissues. Occlusion of sweat ducts with periodic acid-Schiff-positive and Congo red-positive material was noted on microscopic tissue examination. Toll-like receptor 2 was shown to be activated in AD lesional skin (immediately proximal to the sweat ducts), which likely led to the initiation of proteinase-activated receptor 2-mediated pruritus and MyD88-mediated spongiosis. Biofilm formation by AD-associated staphylococci almost certainly plays a major role in the occlusion of sweat ducts and leads to inflammation and pruritus. We believe the environmental hit in AD relates to staphylococci and their biofilms, which occlude sweat ducts.

Fungal Malignant Otitis Externa with Facial Nerve Palsy: Tissue Biopsy Aids Diagnosis

Fungal malignant otitis externa (FMOE) is a serious and potentially life-threatening condition that is challenging to manage. Diagnosis is often delayed due to the low sensitivity of aural swabs and many antifungal drugs have significant side effects. We present a case of FMOE, where formal tissue sampling revealed the diagnosis and the patient was successfully treated with voriconazole, in addition to an up to date review of the current literature. We would recommend tissue biopsy of the external auditory canal in all patients with suspected FMOE in addition to routine microbiology swabs.

Genetic determinants and elements associated with antibiotic resistance in viridans group streptococci

To investigate the distribution of erythromycin, tetracycline and chloramphenicol resistance mechanisms and determinants and the relevant genetic environments and elements in viridans group streptococci (VGS). A total of 263 VGS collected from routine throat swabs in 2010-12 and identified to the species level were studied. Antibiotic resistance determinants and the relevant genetic contexts and elements were determined using amplification and sequencing assays and restriction analysis. The investigation provided original information on the distribution of resistance mechanisms, determinants and genetic elements in VGS. Erythromycin-resistant isolates totalled 148 (56.3%; 37 belonging to the cMLS phenotype and 111 belonging to the M phenotype); there were 72 (27.4%) and 7 (2.7%) tetracycline- and chloramphenicol-resistant isolates, respectively. A number of variants of known genetic contexts and elements carrying determinants of resistance to these antibiotics were detected, including the mega element, Φ10394.4, Tn2009, Tn2010, the IQ element, Tn917, Tn3872, Tn6002, Tn916, Tn5801, a tet(O) fragment from ICE2096-RD.2 and ICESp23FST81. These findings shed new light on the distribution of antibiotic resistance mechanisms and determinants and their genetic environments in VGS, for which very few such data are currently available. The high frequency and broad variety of such elements supports the notion that VGS may be important reservoirs of resistance genes for the more pathogenic streptococci. The high rates of macrolide resistance confirm the persistence of a marked prevalence of resistant VGS in Europe, where macrolide resistance is, conversely, declining among the major streptococcal pathogens.

Colonization and Infection due to Multi-resistant Bacteria in Neonates: A Single Center Analysis

In the last years the prevalence of multi-resistant pathogens (MRPs) has increased. Systemic infections remain important for neonatal morbidity and mortality. Neonates born between January 2011 and December 2012 and admitted to the neonatology before their tenth day of life were included into this retrospective analysis. Vancomycin-resistant Enterococci, Methicillin-resistant Staphylococcus aureus, Gram-negative bacilli with Extend Spectrum Beta Lactamase or AMP-C resistance were defined as multi-resistant pathogens (MRPs). MRP positive and negative patients were analyzed regarding clinical risk factors and the incidence of systemic infections. 635 neonates were admitted during the analysis period. In 31 patients MRPs were detected. 2 patients developed MRP-associated infections. Both were discharged without long term health risks. Low gestational age and need for mechanical ventilation were risk factors for colonization with MRPs in the univariat analysis. The incidence density (per 1 000 patient days) for all MRE increased from 0.76 in 2011 to 3.51 in 2012. In contrast the sepsis rate remained stable (14.9% and 14.2%). 2 MRP colonization clusters were detected by routine microbiology swabs. Both could be controlled by appropriate hygienic measures. The prevalence of Gram-negative MRPs increased in neonates. Microbiological screening seems to be helpful for early detection of colonization and thus prevention of nosocomial infections with MRPs. Despite the increased attention towards the problems associated with multiresistant bacteria, there are still major efforts needed for prevention and early treatment of sepsis with non-resistant bacteria.

Gonococcal Infections

Gonococcal Infections

Gonococcal Infections

Gonococcal Infections

Evaluation of chromogenic meticillin-resistant Staphylococcus aureus media: sensitivity versus turnaround time

Chromogenic media offer the potential for improved detection of meticillin-resistant Staphylococcus aureus (MRSA). Incubation times in a clinical setting vary, generally between 16 and 48h. To investigate the effects of incubation time on the performance of commercial chromogenic MRSA media. We initially compared the early recovery of MRSA isolates from routine screening swabs following 16-23h to that after 22-24h of incubation using ChromID MRSA (bioMérieux). We also compared ChromID MRSA, ChromID MRSA V2 (bioMérieux), Brilliance MRSA 2 Agar (Oxoid) and Colorex MRSA (E&O Laboratories Ltd) as selective media for 6035 MRSA screening swabs. Only 303 of 623 (48.6%) MRSA isolates detected by this medium after 48h were recovered after 16-23h compared with 726 of 1018 (71.3%) isolates after 22-24h. Following implementation of the 22-24h incubation period, 50 (4.4%) isolates exhibiting a positive chromogenic reaction gave conflicting coagulase latex and DNase results. In 88% of these cases, DNase gave the correct result of non-MRSA. ChromID MRSA demonstrated the highest sensitivity overall at 93.2%, followed by Colorex MRSA Agar (87.1%), ChromID V2 (83.7%) and Brilliance Agar (78.2%). All media exhibited specificities of >99.7%. Early detection of MRSA using ChromID MRSA is best achieved after incubation for 22-24h. Latex agglutination coagulase tests alone should not be relied upon for identification of MRSA isolates from this medium. Although ChromID MRSA was deemed the superior medium in terms of MRSA recovery, the disadvantages of increased turnaround time (48 vs 24h) must be considered.

How Useful is Routine Amniotic Fluid and Neonatal Surface Swab Microbiology at Caesarean Section?

Our aim was to evaluate the clinical impact of routine amniotic fluid and neonatal surface swab microbiology at Caesarean section. Microbiology data from 1 537 neonates delivered by Caesarean section were analysed in the light of clinical outcome. 1 340 (87%) neonates had non-pathogenic bacteria or negative culture results from both amniotic fluid and surface swab samples. Of the 197 (13%) neonates with pathogenic bacteria, 22 (1.4%) were diagnosed with infection, but only in 6 (0.4%) were the bacteria presumed to be responsible for the infection. Amniotic fluid and surface swab culture had sensitivities of 54% and 35%, and positive predictive values of 14% and 17%, respectively, for detecting a neonate at risk of infection. Amniotic fluid and neonatal surface swab microbiology at Caesarean section contributes little if anything to postnatal management and can be safely dropped from operative routine.

Detection of human metapneumovirus in infants with acute respiratory tract infection

Both respiratory syncytial virus (RSV) and human metapneumovirus (hMPV), a recently discovered respiratory pathogen classified in the Paramyxoviridae family, have been associated with acute respiratory tract infections (ARTI) in infants. The aim of our study was to determine the incidence and clinical features of hMPV infection in infants presenting with ARTI at a tertiary children's hospital in Greece during the 2007-2008 and 2008-2009 winter seasons. Routine nasal swabs were obtained from 127 infants younger than 12 months of age. The specimens were tested for the presence of hMPV and RSV antigens using validated enzyme-linked immunoassays. The mean age of the studied infants was 3 months (range 19 days to 12 months). hMPV was detected in 8 (6.3%) children, 4 boys and 4 girls, while RSV was detected in 39 (30.7%) children, 22 boys and 17 girls. In 7 children, hMPV was identified as a unique viral pathogen, while only 1 child was co-infected with hMPV and RSV. Bronchiolitis was diagnosed in all hMPV-positive cases, while only two cases required hospitalization and supplemental oxygen therapy. Our results provide further evidence of the importance of hMPV as a pathogen associated with community-acquired ARTI in infants in Greece.

Swabbing-Test Interpretation Using Nonlinear Regression in San Jorge Gulf Basin

Summary In the San Jorge basin in southern Argentina, swabbing tests are the conventional testing technique used by YPF to evaluate production potential on those reservoirs that do not flow naturally to surface. During the well-completion phase, a swabbing device is lowered into the wellbore to test each reservoir layer individually. These wells, which produce from multilayer formations, are later completed for commingled production. Accordingly, the swabbing test represents the only opportunity to measure dynamic properties for each individual layer. So far, pressure-transient analysis of such tests has been limited to the use of conventional interpretation methods applied to the infinite-acting radial-flow (IARF) portion on those tests in which such a flow regime is observed. This can happen if a bottomhole shut-in valve is used or if the shut-in period is given a long enough time for the bottomhole pressure to reach conditions close to initial reservoir pressure. Unfortunately, both practices increase the completion cost substantially, which affects well economics strongly. As a result of this economic limitation, neither practice is applied on a routine basis, so most of the time, transient-pressure interpretation is not performed. In an attempt to obtain value from routine swabbing tests, where an IARF condition is not observed, a different method of test interpretation has been introduced. Through the computation of the instantaneous flow rate from the pressure-trend increase during the swabbing period, it is possible to use nonlinear numerical regression to make an estimation of the reservoir permeability, even if IARF is not present. With this innovative approach, YPF is constructing for the first time representative production models for each well, thereby improving reserves estimations and production forecasts.